Application of Wellbore Strengthening Techniques in Carbonate Formation Solves Lost Circulation Challenges During Liner Running and Cementing

For many years, the oil and gas industry has deployed techniques which enhance formation strength via the successful propping and plugging of induced fractures. Induced fracture sizes have been successfully treated using this method up to the 600 – 1,100-micron range. Static wellbore strengthening techniques are commonly deployed to cover 1,000 micron and all fracture size risks underneath. The deployment of wellbore strengthening techniques has historically been confined to permeable formations. In most cases, wellbore strengthening has been deployed to operationally challenging sand fracture gradients or, where boundaries are pushed, lower ranges of permeability, such as silts. The subject of wellbore strengthening in shales or carbonates to this day, remains a challenge for the industry, with very few documented success stories or evidence of sustained ability to enhance fracture gradient across a drilling campaign. This paper covers the history of lost circulation events which have been reported in the Khazzan/Ghazeer field in the carbonate Habshan formation. It also describes the design changes which were introduced to strengthen the rock and enable circulation/returns, during liner cementation. The design work built on experience applying wellbore strengthening techniques in carbonates in the Norwegian sector of the North Sea. This work is also summarized in this paper. The Habshan carbonate formation in Oman presents a lost circulation challenge through an ‘induced’ fracture risk. Since the beginning of the drilling campaign in the Khazzan/Ghazeer field, the Habshan formation has repeatedly experienced induced mud losses during well activities such as liner running, mud conditioning with liner on bottom and cementing, when the formation is exposed to higher pressures, less so during drilling. The Habshan challenge in Oman has led to regular, significant lost circulation events during cement placement, adding operational cost and more importantly, presenting difficulties around meeting zonal isolation objectives. Through previous field experience in Norway, a set of criteria was developed to qualify a standard pill approach to carbonate strengthening. The currently deployed strategy is designed to address both the risk of induced fracture by propping and plugging (wellbore strengthening) and provide some ability to seal natural fractures which are often encountered with carbonates, or similarly flawed rocks. The strategy deployed aims to cover these two risks with a blanket approach to lost circulation risk in carbonates. The success of this approach is demonstrated using well performance data from a total of 43 wells drilled before and after the introduction of the wellbore strengthening strategy. As it was initially assumed that wellbore strengthening could not be applied to carbonate formations, other techniques had been tried to prevent lost circulation. Those techniques provided mixed results. Since the implementation of wellbore strengthening significant improvements in achieving zonal isolation requirements and reducing fluid losses have been documented.

Impact of Static and Dynamic Wellbore Strengthening on Well Planning in Petroleum Development Oman

Formation Integrity Tests (FIT) or Leak-Off Tests (LOT) are common techniques to reduce the uncertainty in Fracture Gradient (FG) prediction for well planning, but are usually performed at the casing shoe. This article will discuss the first examples of open-hole LOT and FIT in Petroleum Development Oman (PDO), targeting depleted formations in water injector or oil producer wells. The data was used to justify continued drilling of slim wells with two casing strings, where otherwise three casing strings would be required, provided dynamic wellbore strengthening is applied. In addition, the concept of static wellbore strengthening was also trialed for the first time in Oman, using the hesitation squeeze testing procedure, by which the effective leak-off pressure was incrementally increased to match the maximum ECD required for cementing.

Block 61 Drilling Fluids Optimization Journey

BP operates Khazzan & Ghazeer fields in the Sultanate of Oman with the aim to deliver safe, reliable and efficient wells. Efficiencies within drilling fluids design form part of a greater continuous improvement cycle to well delivery cost. With fluids spend contributing to a significant portion of the executed well cost (typically 15 % in Oman), fluids design changes hold the potential to yield positive cost savings (where well performance is maintained). This paper presents the areas of fluids design which were explored to reduce fluids spend as part of the continuous improvement cycle. Combined, the changes to fluids design evolved to reduce the fluids cost of Barik vertical wells to 6% of total well cost. All avenues of fluids design and the costs associated with the fluids operation in Oman were viewed as being in scope for change to maintain overbalance hydrostatic pressure on fluids spend. The methodology employed to reduce fluids spend can be described in four steps as per continuous improvement roadmaps; identify the cost saving project, the key enablers which allow the cost saving to be realized, risk/reward analysis where low risk/high reward projects were accelerated as priority and placed to the front of the queue for field trial and where a trial outcome is positive, the change is introduced permanently to the operation. This process worked well in continuously pushing fluid performance and reducing the fluids spend in Oman. The scope of change to fluids design was wide, with each ‘value adding project’ providing its own cumulative cost benefit. The projects which contributed to significantly reducing the overall fluids spend in Oman focused on personnel, fluid type selection, fluids formulation optimization, wellbore strengthening, fluid consumption and recycling, drilling fluids practice and brine selection. Reductions in fluids spend were accompanied with an improved well performance. Well delivery times being continuously observed to improve throughout the campaign (63 days vs 42 days). Whilst the fluids design is not directly responsible for this outcome, it does highlight that the changes made to fluids design positively influenced the improved well delivery performance. The drilling fluids optimization initiatives resulted in significant time and cost saving thus reduction in overall Barik vertical well drilling cost. Drilling fluids cost is reduced by over 55% without impact on safety and drilling performance.

Investigation on the Effect of Mud Additives on the Gelation Performance of PAM/PEI System for Lost Circulation Control

Lost circulation is one of the most common problems in the drilling of oil and gas wells where mud escapes through natural or induced fractures. Lost circulation can have severe consequences from increasing the operational cost to compromising the stability of wells. Recently, polymeric formulations have been introduced for wellbore strengthening purposes where it can serve as Loss Circulation Materials (LCMs) simultaneously. Polymeric LCMs have the potential to be mixed with drilling fluids during the operation without stopping to avoid non-productive time. In this study, the significance of most common conventional mud additives and their impact on the gelation performance of Polyacrylamide (PAM) / Polyethyleneimine (PEI) has been investigated. Drilling fluid with typical additives has been designed with a weight of 9.6 ppg. Additives including bentonite, barite, CarboxyMethylCellulose (CMC), lignite, caustic soda, desco, and calcium carbonate has been studied individually and combined. Each additive is mixed with the polymeric formulation (PAM 9% PEI 1%) with different ratios, then kept at 130°C for 24 hrs. Rheological performance of the mature gel has been tested using parallel plate geometry, Oscillatory tests have been used to assess the storage Modulus and loss modulus. Moreover, the gelation profile has been tested at 500 psi with a ramped temperature to mimic the reservoir conditions to obtain the gelation time. The gelation time of the polymer-based mud was controllable by the addition of a salt retarder (Ammonium Chloride), where a gelation time of more than 2 hours could be achieved at 130°C. Laboratory observations revealed that bentonite and CMC have the most effect as they both assist in producing stronger gel. While bentonite acts as a strengthening material, CMC increases the crosslinking network. Bentonite has successfully increased the gel strength by 15% providing a storage modulus of up to 1150 Pa without affecting the gelation time. This work helps in better understanding the process of using polymeric formulations in drilling activities. It provides insights to integrate gelling systems that are conventionally used for water shut-off during the drilling operation to replace the conventional loss circulation materials to provide a higher success rate.

Mitigating Well Construction Challenges of a Caspian Deepwater Exploratory Well: A Cementing Perspective

BP returned to Caspian deepwater exploratory drilling in 2019. The exploration well was drilled on the Shafag-Asiman structure in water depths greater than 2,000 ft. Well challenges included high shallow water flow (SWF) risk with multiple re-spuds on the nearest offset, lost circulation due to complex wellbore geometry combined with a narrow pore and fracture gradient window, and uncertainty in pore pressure prediction in abnormally pressured formations with a new depositional model. In addition, a well total depth more than 23,000 ft, eight string casing design and bottom-hole pressures greater than 20,000 psi presented a truly modern-day challenge to well integrity. A six-month planning phase for the cementing basis of design concluded by delivering slurry designs capable of combating SWF, qualified by variable-speed rotational gel strength measurement. Engineered lost circulation with selective placement of wellbore strengthening materials in combination with cement and mechanical barriers to provide isolation and integrity for the life of the well. Exhaustive pilot testing to account for changes required a cement design based on pore pressure variation and comprehensive modeling for hydraulics, centralizer placement, and mud displacement. This was complemented by a custom centralizer testing process specifically designed to simulate forces exerted in wells with similar complexity. Long-term effects on cement were evaluated, not only for placement but also for future operations including pressure and temperature cycles during wellbore construction or abandonment.

Wellbore Strengthening Design Criteria and Enhanced Fracture Gradient for Widening Stable Mud Weight Window and Enabling Safe and Efficient Drilling in Depleted Reservoirs

Drilling in permeable formations, especially depleted reservoirs, can particularly benefit from simultaneous wellbore shielding and strengthening functionalities of drilling mud compounds. The ability to generate simultaneous wellbore shielding and strengthening in reservoirs has potential to widen stable mud weight windows to drill such reservoirs without the need to switch from wellbore strengthening compound to wellbore shielding compound, and vice-versa. Wellbore shielding and strengthening experiments were conducted on three outcrop sandstones with three mud compounds. The wellbore shielding stage was conducted by increasing the confining and borehole pressures in 4-5 steps until both reached target pressures. CT scan images demonstrate consistency of the filtration rates with observed CT scanned mud cakes which are dependent on the sandstone pore size and mud compound particle size distributions. In wellbore strengthening stage, the borehole pressure was increased until fracture was initiated, which was detected via borehole pressure trend and CT scan imaging. The fractures generated were observed to be plugged by mud filter solids which are visible in the CT scan images. The extent of observed fracture solid plugging varies with rock elastic properties, fracture width and mud compound particle size distribution. Based on the laboratory test data, fracture gradient enhancement concept was developed for the mud compounds. In addition, the data obtained and observations from the tests were used to develop optimal empirical design criteria and guidelines to achieve dual wellbore strengthening and shielding performance of the mud compounds. The design criteria were validated on a well which was treated with one of the mud compounds based on its mud loss events during drilling and running casing.

Pushing the Frontier in Deepwater HP/HT Drilling by Application of Wellbore Strengthening—A Practical Approach

This paper presents a comprehensive laboratory and field study, discussing the development, formulation, and application of a wellbore strengthening mechanism, for strengthening weak formations while drilling in a deepwater high-pressure/high-temperature (HP/HT) well environment. The use of this technology has potential to eliminate nonproductive time (NPT) related to downhole losses, along with extending the drillability of sections and eliminating additional casing strings, during exploratory drilling. During the planning phase of a sequence of deepwater and HP/HT exploration wells, the potential high-pressure case scenario drove the planned and contingency well casing designs. This led to an extensive casing program with a 16-in. sub mudline hanger casing string added to the base design, as well as the normal 36-in. conductor, 20-in. surface casing, 13 ⅜-in. intermediate casing, and 9 ⅝-in. casing, which would enable reaching total depth (TD) within a planned 8 ½-in. hole. The realistic offset well driven by the high-pressure case also required two further contingency liner strings (11 ¾ in. and 7 in.), to be included in the well design. A key enabler for the sequence of wells was that the semisubmersible rig was upgraded to include a managed pressure drilling (MPD) below tension ring (BTR) arrangement. This was enhanced by the MPD well control system and associated risk assessment, allowing working to reduced acceptable kick tolerance limits. In addition to the outlined base and contingency plans, wellbore strengthening was also to be available, as an additional contingency application, to reach TD objectives. Thus, extensive laboratory tests were performed for wellbore strengthening design, using proprietary software, along with past established practices. Subsequent to laboratory testing and the optimal formulation, a detailed wellbore strengthening program was prepared and included in the drilling program, for potential use at any point while drilling ahead. On one well, after cementing of 13 ⅜-in. casing and performing a leakoff test (LOT), it was found that the value was insufficient for drilling through the entire planned section. A contingency 11 ¾-in. liner was being enabled before it was decided to pump the wellbore strengthening pill and strengthen the casing shoe. The pill application gave sufficient increased formation strength, leading to the well section being successfully drilled and cased with no losses, even though the high-pressure well scenario was actually encountered. This solution eliminated the time and cost implication and considerable operational challenges of the 11 ¾-in. contingency liner. This paper presents the study of conceptualizing the wellbore strengthening mechanism and implementing this customized solution in the field. A detailed analysis is also done to identify the optimal products, compatibility with drilling fluid, formation and existing chemical permit, and cost-effectiveness and savings using wellbore strengthening practice. The paper also discusses the comprehensive pit management program and required treatment plan while drilling.