A review of the various treatments of oil-based drilling fluids filter cakes

Treatment of the filter cake layer after drilling is essential for better cement integrity and to retain the original reservoir permeability. Compared to water-based filter cake, oil-based mud filter cake removal is more sophisticated as oil encloses the filter cake’s particles. Therefore, oil-based mud clean-up requires wettability alteration additives (mutual solvents and/or surfactants) for permitting acid/filter cake reaction. With an appropriate acid, microemulsions were reported to be very efficient in cleaning oil-based filter cakes, due to their low interfacial tension and high acid solubility. The objective of this paper is to provide an overview of the different techniques and treatment solutions utilized in oil-based filter cake clean-up. Furthermore, a synopsis of the various treatments for drilling fluids densified with different weighting materials is presented. Subsequently, the research limitations and opportunities have been highlighted for future work. In the light of the review that has been presented in this paper, it's recommended to conduct further investigation on some areas related to filter cake removal. The removal of filter cake formed from weighting materials other than barite, calcium carbonate, ilmenite, and manganese tetroxide needs to be investigated thoroughly. Additionally, the overall efficiency of oil-based mud removal needs to be studied under wide ranges of temperature, salinity, and pH. The utilization of surfactant-free microemulsions in filter cake treatment could also be investigated.

Exploiting Slim Pulsed Neutron Spectroscopy for Unlocking Reservoir Potential in Brownfields: Two Examples from Gulf of Suez Offshore Field in Egypt

For more than 40 years, pulsed neutron spectroscopy has been primarily used in reservoir management to determine hydrocarbon saturation profiles, tracking reservoir depletion, and planning workover activities to diagnose production problems such as water influx. Legacy pulsed neutron tools used to provide this information for more than four decades, but they were challenged when a mixed lithology reservoir is encountered, complex completions, unknown borehole conditions, and poor cement integrity in cased boreholes. This paper presents two successful field examples and applications using the advanced slim pulsed neutron spectroscopy to precisely determine multiphase contacts in a complex geological structure, provide current hydrocarbon saturation independent of the quality of cement behind the casing, and identifying bypassed hydrocarbon. This was of paramount importance in understanding current reservoir fluid distribution to reveal the true potential of this offshore brownfield located in the Gulf of Suez, Egypt. An integrated approach and candidate well selection were done that resulted in selecting two candidate wells that had poor cement quality behind casing, heterogeneous carbonate reservoir with mixed lithology, and uncertain fluid contacts in a complex reservoir structure. These combined borehole and reservoir conditions resemble challenges for capturing this crucial information with high confidence using the legacy pulsed neutron tool, and therefore required an advanced technology that can overcome these challenges using a single logging mode at twice the logging speed of any current pulsed neutron technology available in the industry. Based on the results, a workover campaign was implemented in this mature field to increase overall oil production with very efficient cost control, especially with this unprecedented time the O&G industry is going through. An integrated approach was set that resulted in the selection of two wells for the saturation determination logging tool deployment. Detailed high-resolution mineralogy, self-compensated total porosity and sigma, fluid type identification, and multiphase fluid saturation was obtained with high precision behind cased borehole independent of cement integrity and borehole fluid reinvasion. The results provided crucial information as an input to the integrated reservoir engineering approach which revealed around a 100-m net oil interval which was previously overlooked due to relatively low resistivity. Besides, fluids contacts were evaluated that confirmed the development of a secondary gas cap and the water encroachment direction. This technology can be further applied to more brownfields provided the right candidate selection is done to understand the potentiality of the field which would increase the recovery factor of the brownfields that represent almost more than 65% of the oil and gas fields around the world.

Cement Conundrum: Valuable Lessons Learned for Sustaining Production

Addressing wellbore integrity through cement evaluation has been an evergreen topic which frequently catches major operators by surprise due to premature water or gas breakthrough causing low production attainability from the wells. Managing idle well strings arising from integrity issues is also a challenge throughout the production period. The remedial solutions to these issues do not come conveniently and require high cost during late life well intervention which often erodes the well economic limit. A critical element of wellbore barrier which is cement integrity evaluation is proposed to be uplifted and given a new perspective to define success criteria for producer wells to achieve certain reserves addition and production recovery. This paper will highlight integrated factors affecting cement bond quality, impact to well production, potential remedies for poor cement bond observed leveraging on the enhanced workflow and new technology and way forward to proactively prevent the unwanted circumstances in the first opportunity taken. A set of recommendations and prioritization criteria for future cement improvement will be also highlighted. Several case specific wells logged with variable cement bond evaluation tools are re-assessed and deep-dived to trace the root causes for unsatisfactory cement bond quality observed which include reservoir characteristics, understanding anomalies during drilling and cementing operation, identifying cement recipe used, log processing parameters applied and observing best practices during cementing operation to improve the quality. New and emerging cement evaluation technology inclusive of radioactive-based logging to meet specific well objectives will be also briefly discussed in terms of differences and technical deliverables. Looking at each spectrum, results show that there are several interdependent factors contributing to poor cement bond quality observed. Accurate understanding of formation behavior, designing fit-for-purpose cement recipe and adequate planning for cementing operation on well-by-well basis are among the top- notch approaches to be applied for an acceptable cement bond quality and placement. Statistics show that 27% to 64% of production attainability is achieved by wells with good cement quality within the first 3 months of production and this increases to 85% to 98% up until 7 months of production period, while only 12% production attainability achieved for those wells with adverse cement quality issue. In another well, water cut as high as 47% since the first day of production is observed which keeps increasing up to 40% thereafter. In a nutshell, cement evaluation exercise shall not be treated as vacuum, instead it requires an integrated foundation and close collaboration to materialize the desired outcomes. Arresting the issue with the right approach in the first place will be the enabler for optimum well performance and productivity to exceed the recovery target.

Cement Evaluation in Highly Laminated and Multi-Stacked Sandstone Reservoirs: The World First Novel Approach Using Dual-Physics Cement Bond Assessment

Advances in cement recipe, additives and cementing technology including light weight cement, ultra-low fluid loss cement blend and improved cement to mud rheology mixing to seal the continuous liquid channels have prompted the industry to find an innovative way to evaluate the cement bond and integrity with a more robust and integrated approach. Evaluating cement bond behind casing based on single tool platform had shown some inherent uncertainties mainly due to borehole effects, tool eccentralization and processing variation. This paper will highlight few case studies on the application of both electromagnetic acoustic wave (EMAT) and ultrasonic cement evaluation logs including the world's first tool combination in single run to enhance understanding on cement integrity and optimize the perforation interval for production. Channeling and microannulus occurrences whether dry or wet are the most common features in cement integrity evaluation and yet poorly characterized to prevent any unwanted cross-flow or adverse impact to production. Electromagnetic acoustic wave cement evaluation in combination with an ultrasonic tool allow direct quantification of compressional, shear and flexural attenuation properties of cement downhole as well as acoustic impedance and microdebonding feature of the cement. Separation between average shear and flexural attenuation curves may indicate presence of microannulus depending on the extent of the separation without any requirement of additional pressurized logging pass. Parameter threshold determination based on shear and flexural attenuation cross-plot also indicates severity of cement microdebonding. Results showed that good production rate with lower water cut and low GOR reading had been achieved from specific perforated zones in the well. Electromagnetic acoustic wave and ultrasonic cement evaluation tools had successfully defined the zonal isolation layers as thin as 2 to 3 meters along the wellbore and optimized the perforated zones to avoid any liquid channeling or premature water and gas breakthrough into the wells, which can affect the production attainability and drainage efficiency from particular reservoirs. In a nutshell, combination of EMAT acoustic wave and ultrasonic cement evaluation principles prove to provide a more comprehensive overview on the cement bond integrity behind the casing. Having two independent downhole measurement which complement each other will reinvent the effort in cement bond assessment for complex reservoir environment which is susceptible to interpretation ambiguity.

First Abu Dhabi 9.625in × 12.25in Non-Directional Casing While Drilling CWD Run for Intermediate Hole Section Saves Two Days Rig Time, Enhancing Drilling Efficiency & Improving Well Integrity

Significant mud losses during drilling often compromises well integrity whenever sustainable annular pressure (SAP), is observed due to poor cement integrity around 9-5/8-in casing in wells requiring gas lift completion. Heavy Casing Design (HCD) is applied as a solution; whereby, two casing strings are used to isolate the aquifers and loss zones, thus ensuring improved cement integrity around the 9 5/8-in intermediate casing. Casing While Drilling (CWD) is a potential solution to mitigate mud losses and wellbore instability enabling an optimized alternative to HCD by ensuring well integrity is maintained while reducing well construction cost. This paper details the first 12 ¼-in × 9-5/8-in non-directional CWD trial accomplished in Abu Dhabi onshore The Non-Directional CWD Technology was tested in a vertical intermediate hole section of a modified heavy casing design (MHCD) aimed at reducing well construction cost over heavy casing design (HCD) as shown in the figure 1. A drillable alloy bit with an optimized polycrystalline diamond cutters (PDC) cutting structure was used to drill with casing through a multi-formation interval with varying hardness and mechanical properties. Drilling dynamics, hydraulics and casing centralization analysis were performed to evaluate the directional tendency of the drill string along with the optimum drilling parameters to address the losses scenario, hole cleaning, vibration, and maximum surface torque. The CWD operation was completed in a single run with zero quality, health, safety, and environment (HSE) events and minimum exposure of personal to manual handling of heavy tubulars. Exceptional cement bonding was observed around the 9 5/8 in casing indicative of good hole quality despite running a significant number of centralizers (with smaller diameter), compared with the conventional drilled wells (cement bond logging was done after the section). CWD implementation saved two days of rig operations time relative to the average of the offset wells with the same casing design. The rate of Penetration (ROP) was slightly lower than the conventional drilling ROP in this application. The cost savings are mainly attributed to the elimination of casing-running flat time and Non-Productive Time (NPT) associated with clearing tight spots, BHA pack-off, wiper trips. The application of CWD in the MHCD wells deliver an estimated saving of USD 0.8MM in well construction cost per well compared to the HCD well design. Additional performance optimization opportunities have been identified for implementation in future applications. The combination of the MHCD and CWD technology enhances cementing quality across heavy loss zones translating into improved well integrity. Implementing this technology on MHCD wells could potentially save up to USD 200MM (considering 250 wells drilled). This is the first application of the technology in Abu Dhabi and brings key learning for future enhancement of drilling efficiency. The CWD technology has potential to enhance the wellbore construction process, which are typically impacted by either circulation losses and wellbore instability issues or a combination of both, it can applied to most of the offshore and onshore fields in Abu Dhabi.

Case Studies on Multistring Isolation Evaluation in P&A Operations

Cement sheath quality assessment is a critical initial step in plug and abandonment (P&A) operations during oil and gas well decommissioning. However, the technologies commonly used require unimpeded access to the casing annuli, thus enforcing the need for production tubing pulling or inner casing milling. Cement integrity or isolation evaluation through multilayered well casing strings will provide the opportunity to significantly reduce operational time and costs and to greatly simplify the traditional P&A process. As desired by the industry for years, recent advancements in isolation evaluation have proven the feasibility to assess cement sheath quality without the removal of production tubing or inner casing. The new development, consisting of a sophisticated logging apparatus with a novel processing methodology, led to a groundbreaking technology evaluating zonal isolation through multiple casing strings in wells. The logging tool is deployed in the borehole using E-line, slickline, or coiled tubing. Then, the acoustic energy that is emitted and received by the tool travels through the tubing and surrounding annulus to reach the isolation barrier behind the casing. A proprietary frequency-domain processing algorithm successfully identifies the desired signal by discriminating it from overwhelming undesired signals such as tubing arrivals. The latest development stage further enables the segmentation of the measurements, providing an improved sensitivity to detect the azimuthal variations in the cement sheath quality. Case histories of applying omnidirectional and segmented multistring isolation evaluation technology in field trials in the North Sea are presented in the paper. The measurement accuracy has been verified through side-by-side comparisons with industry-standard cement bond log (CBL) and ultrasonic logs recorded after the tubing was removed. Additionally, the technology has been proven applicable to various casing or tubing weight and size combinations with tubing eccentric inside the casing. Thus, it is practicable in actual well configurations and suitable for the deviated well sections as well. In conclusion, this innovative technology that exhibits quantitative assessments of bonding or isolation conditions of wells in multistring configurations provides a cost-effective solution during P&A and further demonstrates a great potential to accelerate along the path to a rigless P&A operation.

Optimisation of Plug and Abandonment Process Utilising Nuclear Technology for through Tubing Cement Evaluation

One of the major factors in the well plug and abandonment (P&A) process is to provide a proper isolation in aging wells which requires effective placement of the cement plug in the most suitable location in the well. Identifying cement placement is usually achieved by running cement evaluation logging to define the quality of cement and top of cement depth behind and in between the casing annuli. However, this comes with significant costs due to tubulars or casings removal requirement prior to logging run in order to conduct a proper evaluation. This is necessary since acoustic and ultrasonic based cement evaluation technologies will not be able to determine cement quality behind several casing layers if the job is done through tubing. The cost involved is substantial especially in offshore operation in which the daily operating rate is significantly higher compared to an onshore operation. A new approach to cement evaluation has been tested during the well P&A campaign in one of the aging oil fields in offshore, Peninsular Malaysia. A nuclear based technology comprised of Gamma-Gamma, Neutron-Neutron and Neutron-Gamma measurements were utilized to evaluate cement integrity behind production casing and between production casing as well as intermediate casing while logging run was deployed through tubing in memory mode. Log data was compared with acoustic and ultrasonic based cement evaluation technology that was deployed after the tubing was pulled out in one of the wells. Results had shown a consistent finding with the conventional ultrasonic based cement evaluation data. Based on the logging results, cement placement design and depth was finalized and the cement plug was successfully tested as outlined in the well P&A guideline. Findings from this logging run had provided useful insight to the operator to validate the nuclear based thru tubing cement evaluation technology for wells P&A application. Huge cost saving could be captured through this application as a result of eliminating total rig days via offline cement evaluation logging and based on the results obtained planning for the exact well P&A design requirement prior to the rig arrival. This paper will outline the method, tools used to acquire the cement evaluation data and its operational advantages. Acquired data will be presented and discussed along with the methodology used to determine cement volume and top of cement depth behind and in between the casings.

Slim Pulsed Neutron Spectroscopy Lands for the First Time in the Egyptian Western Desert to Provide a Complete Formation Evaluation – A Novel Case Study from Abu Sennan Field

Wellbore stability issues associated with mechanical failure of the formations frequently present a challenging environment for running openhole logs. Alternatively, casedhole logs can be used to provide multiple physical properties of the formation to help in reservoir characterization. Generally, conventional casedhole measurements have limitations due to the effect of borehole fluids as well as cement integrity. Therefore, it can be challenging to complete an accurate full reservoir evaluation using conventional casedhole measurements. In a field example from the Western Desert, Egypt, a state-of-the-art advanced slim pulsed neutron technology was deployed for the first time to provide a comprehensive standalone formation evaluation without openhole data. When hydrocarbon exploration and development move into more challenging environments, deployment of fit-for-purpose technologies is required. The advanced slim pulsed neutron processing algorithms were used for recording capture and inelastic elemental spectroscopy for rock elemental concentrations, including total organic carbon, detailed mineralogy, and matrix properties, simultaneously with sigma and other neutron-based outputs. By integrating the independent pulsed neutron measurements that are borehole self-compensated, casedhole reservoir characterization is now feasible with high accuracy and precision where the conveyance of openhole logging is of high risk due to borehole conditions and wellbore stability issues. This case demonstrates the applicability of advanced slim pulsed neutron logging for comprehensive reservoir characterization in casedhole environments without any openhole data. It presents this innovative approach for the first time in the Egyptian Western Desert in a field with complex geological background, mixed lithology, and reservoir fluids types. The characterization paved the way to a new gas discovery in a complex clastic environment with a total net pay of 36m of gas condensate. Detailed mineralogy, reservoir porosity, fluid types identification, and hydrocarbon saturation were obtained with high accuracy behind the cased borehole independent of cement integrity and borehole fluid invasion. Leveraging this novel approach, the same methodology can be applied to old reservoirs with limited openhole data available to provide a robust formation evaluation that would aid in re-assessing brownfields. The advanced slim pulsed neutron spectroscopy establishes a paradigm shift in reservoir characterization for casedhole environments to provide a comprehensive formation evaluation and fluid saturation without any openhole input. The workflow can be implemented in various scenarios as a cost-effective solution for reservoir evaluation or reservoir management applications.

Advanced Cement Mechanical Integrity for Thermal Wells

Ensuring wells’ cement mechanical integrity (CMI) is of paramount importance for the success of a thermal project. Failed cement sheaths can lead to loss of production, environmental pollutions, or even to well abandonment. Over time, CMI software applications have been developed to design wells that do not leak. However, their efficiency depends not only on if their equations are verified, but also on how the models are validated versus wells’ downhole conditions. Unfortunately, most CMI tool designers have focused on only verifying if the models are mathematically correct, checking what is the time required for a simulation, and improving how are the simulations reported to the user. Typically, little time is dedicated on validating that the correct model is used for the specific well. This foresight has led to non-predictive CMI tools, which do not allow optimizing well designs. The authors have been involved for more than 15 years in developing and validating CMI models. They have shown the importance of simulating the cement hydration to evaluate the state of stress in the cement after it has set. They also have highlighted how the plastic behavior of the cement design can lead to opening micro-annuli at the cement-sheath's interfaces. Recently the authors have started theoretical work in the area of the cement integrity of high and ultra-high temperature wells and how these temperatures, either naturally occurring or induced, could affect the cement's mechanical integrity. The work has focused on modeling the increase in pore pressures, the opening of micro-annuli at the cement sheath's boundaries, and the phase changes which take place in the cement when it is heated to high temperature values. To date this work showed that heating cement up to 250°C can result in pore pressures larger than 100 MPa unless if the pore pressures can be released. This work has also identified three mechanisms that can lead to such release of pore pressures: 1) During cement hydration, due to the water consumption by the chemical reactions, 2) When a micro-annulus opens due to the large pore pressures, therefore allowing venting the pressures to the surface or to a downhole reservoir, and 3) When a change of phase occurs in the cement when heated to more than 110°C, as this leads to the creation of additional porosity in the cement. All this means that the cement sheath should not be simulated as a closed system, but rather as an open thermo-hydro-chemo-mechanics. How these features impact CMI has never been studied before even if they can explain why some cement designs lead to tight cement sheath and other to leaking ones. This paper highlights the work that has been done and when these conditions should be considered, and if it is feasible to design cement sheaths that do not fail, even at very high temperatures.

A Curious Case of Addressing Well Integrity Issue with Production Logging

The productive life of a well can be affected by deterioration of the well integrity which can be due to casing/tubing corrosion, casing damage during drilling/ work over, packer failure, plug failures, cement integrity issues etc. Remedial measures can be executed if the nature of problem is diagnosed. One can receive early warning of a potential problem and obtain data for determining a specific restoration program by running well integrity diagnostic tools. There are various well integrity tools available to cater evaluation needs at present times with different working principles and targeting different well integrity problems such as casing/tubing and cement integrity. However, in challenging situation and complex environments, the tools may not provide complete diagnostics. Production logging can be an effective tool in such scenarios by mapping the flow behaviour in the wellbore and can provide a better idea of the wellbore problems. The well "A" is a development well which was drilled (max angle~ 27º) and completed in the interval X345-X349m and X362-X368m to exploit gas from the reservoir ABC. During initial testing it produced @ 1, 67,000 m3/d with FTHP of 2348psi. Later after acid job rate increased to 2,20,000 m3/d but later well had frequent water loading problem and required frequent activation / stimulation. The well has good reservoir zones as identified on the OH logs, hence, to diagnose the reason for water production, PLT was planned in the well in 2015. The well was producing 1,00,000 m3/d of gas at an FTHP of 1181psi during that time. Annulus pressure build-up was also observed in the well suggesting integrity issue with packer/tubing. During the PL run, it was observed that the packer has fallen and settled across one the perforation and the flow was observed to be ongoing from inside as well as outside the packer element making the flow interpretation tricky. A proper interpretation was carried out taking into consideration all available data and water entry point was confirmed. Based on the results, a well intervention was carried out and after the job, well started producing 1,50,000 m3/d of gas with 0% water cut whereas before it was producing 1,00,000 m3/d of gas with 1900 BPD of water. Thus, the intervention resulted in production enhancement by 50% and reduction in water cut by 100%. This paper highlights the proper analysis of the recorded data for diagnosis of the flow condition in an adverse and complex scenario and finding out the water entry point for a proper remediation of the well integrity and production issue.