The Evaluation and Introduction of Novel Acid-Based Crosslinked Gel for HPHT Acid Fracturing Applications in Carbonate Formations

Acid Fracturing has been one of the most effective stimulation technique applied in the carbonate formations to enhance oil and gas production. The traditional approach to stimulate the carbonate reservoir has been to pump crosslinked gel and acid blends such as plain 28% HCL, emulsified acid (EA) and in-situ gelled acid at fracture rates in order to maximize stimulated reservoir volume with desired conductivity. With the common challenges encountered in fracturing carbonate formations, including high leak-off and fast acid reaction rates, the conventional practice of acid fracturing involves complex pumping schemes of pad, acid and viscous diverter fluid cycles to achieve fracture length and conductivity targets. A new generation of Acid-Based Crosslinked (ABC) fluid system has been deployed to stimulate high temperature carbonate formations in three separate field trials aiming to provide rock-breaking viscosity, acid retardation and effective leak-off control. The ABC fluid system has been progressively introduced, initially starting as diverter / leak off control cycles of pad and acid stages. Later it was used as main acid-based fluid system for enhancing live acid penetration, diverting and reducing leakoff as well as keeping the rock open during hydraulic fracturing operation. Unlike in-situ crosslinked acid based system that uses acid reaction by products to start crosslinking process, the ABC fluid system uses a unique crosslinker/breaker combination independent of acid reaction. The system is prepared with 20% hydrochloric acid and an acrylamide polymer along with zirconium metal for delayed crosslinking in unspent acid. The ABC fluid system is aimed to reduced three fluid requirements to one by eliminating the need for an intricate pumping schedule that otherwise would include: a non-acid fracturing pad stage to breakdown the formation and generate the targeted fracture geometry; a retarded emulsified acid system to achieve deep penetrating, differently etched fractures, and a self-diverting agent to minimize fluid leak-off. This paper describes all efforts behind the introduction of this novel Acid-Based Crossliked fluid system in different field trials. Details of the fluid design optimization are included to illustrate how a single system can replace the need for multiple fluids. The ABC fluid was formulated to meet challenging bottom-hole formation conditions that resulted in encouraging post treatment well performance.

Multimodal Carbonates: Distribution of Oil Saturation in the Microporous Rock Regions

Improved oil recovery from tight carbonate formations may provide the world with a major source of lower-rate power over several decades. Here we provide an overview of the Arab D formation in the largest oil field on earth, the Ghawar. We investigate the occurrence of microporosity of different origins and sizes using scanning electron microscopy (SEM) and pore casting techniques. Then, we present a robust calculation of the probability of invasion and oil saturation distribution in the nested micropores using mercury injection capillary pressure data available in the literature. We show that large portions of the micropores in Arab D formation would have been bypassed during primary drainage unless the invading crude oil ganglia were sufficiently long. Considering the asphaltenic nature of oil in the Ghawar, we expect the invaded portions of the pores to turn mixed-wet, thus becoming inaccessible to waterflooding until further measures are taken to modify the system’s chemistry.

Saturation and Sigma Measurements in Carbonate Formations with Multiple Pulsed Neutron Technologies: Case Study from Southeast Turkey

This paper describes the petrophysical analysis resulting from operation of two independent pulsed neutron logging tools in the same cased hole well. The well was primarily carbonate and included many different subsurface formations located in the Southeast Anatolia Region of Turkey that included the Derdere, Karababa A, B, and C, Karaboğaz, Bozova, and Germav. Computing the mineralogy and saturation in these environments is challenging due to the complexity and low porosity of the formations that included mixed lithologies and organic shale. One of the objectives of this work was to demonstrate how the spectral data from the two tools was not only consistent, but that they could be combined to create an optimal petrophysical interpretation of the lithology, detailed mineralogy, porosity, and saturation of the formations within the well. Both tools employed a pulsed neutron generator capable of emitting 2 x 108 neutrons/second into the ambient formation. One was a 4-detector, 1-11/16-inch diameter reservoir evaluation tool, and the other was a single detector, 3-1/4-inch geochemical spectroscopy tool. In order to obtain the best possible results, a sound logging program was created that involved running the reservoir evaluation tool in 3 different modes of operation. This included the carbon/oxygen (C/O) mode, the sigma mode, and the gas mode. Stationary measurements were also obtained. The geochemical logging tool has only a single mode of operation. The resulting sigma measurements were in complete agreement. The sigma from the geochemical logging tool was corrected for the effects of diffusion. The advantage of the slim-hole reservoir evaluation tool is that the measurements from the 4th detector are diffusion-free. Data from the 1-11/16-inch reservoir evaluation tool from the gas mode did not reveal any bypassed gas zones in the well. Oil saturation was computed with the reservoir evaluation tool based upon three logging passes in the C/O mode. An important component of the interpretation was that it was supported by MCNP modeling that predicted the tool's response for hydrocarbon saturation. Although data from the geochemical spectroscopy tool was not used to determine saturation in this well, the resulting carbon concentration, that included kerogen as well as hydrocarbons, was completely consistent with the saturation computed from the reservoir evaluation tool.

Geochemistry of natural gases from sediments of terrigenous and carbonate formations of the Bukhara-Khiva oil and gas region of Uzbekistan

The results of long-term studies of natural gases in the Bukhara-Khiva oil and gas region of Uzbekistan are summarized. The results of studying the composition of gases in hydrocarbon deposits generated by OM of continental (terrigenous deposits of the Cretaceous and Middle Jurassic) and marine (carbonate Jurassic) facies are presented. Regularities of changes in individual constituents of gases (hydrocarbons, hydrogen sulfide, carbon dioxide, nitrogen, etc.) with depth are considered; the influence exerted on the composition of gases by various factors and, above all, those of them, which, in the opinion of most researchers, are the main ones. The revealed patterns are used for predictive assessment of the distribution of various constituents of natural gases in the study area.

Water Erosion in the Moulouya Watershed and its Impact on Dams' Siltation (Eastern Morocco)

The Moulouya watershed presents high risks of water erosion. The dams built on this river lose each year, by siltation, a volume of water of about 10 M m3 (Lahlou, 1994). This worrying rate shows that water erosion can hinder the socio-economic development of the basin if soil protection and siltation measures are not taken. This study defines the areas most threatened by water erosion to protect them as a priority. Using a GIS and the Universal Earth Loss Equation (USLE), we mapped soil losses. The adopted model determines soil loss from erosivity of rainfall, soil erodibility, inclination and slope length, soil cover, and erosion control practices. The possible origins of the deposits alluviating the dams are related to the products of alteration of the Secondary's carbonate formations and the Tertiary and Quaternary formations. The modalities of the transport of deposits of banks of the network of Moulouya are mainly by a suspension. The specific degradation in the Moulouya watershed is estimated at 214 t/km2/year. Precipitation intensity, steep slopes and deforestation are responsible for these losses.

Injection of Low-Salinity Water as an Integral Part of Enhanced Oil Recovery Programmes for Carbonate Formations of the Central-Khoreiver Uplift Oilfields

The paper describes the process of evaluation of low salinity water composition to improve the development of hydrophobic carbonate formations of Central-Khoreiver Uplift (CKU) fields with relatively high oil viscosity (5-15 mPa·s) and average formation temperature 70°C. The sources of low salinity water were determined, prospective composition for water injection were analyzed. The efficiency of oil displacement by formation water and low salinity water are observed during the spontaneous imbibition experiments and coreflood tests to compare the efficiency of formation and low salinity water. The expected incremental displacement efficiency for the target carbonate formations can vary widely, from 1 to 10%. Linear models of the completed coreflood tests and a sector hydrodynamic model of the prospective trial injection are built, considering the basic chemical processes while mixing different types of water. We also review the prospects of joint application of low salinity water injection and chemical EOR methods.

Development and Successful Field Trial of Retrievable, Instrumented & Tandem Downhole Isolation Valve RIT-DIV System

This paper describes the development and field deployment of a new downhole isolation valve system called the Retrievable, Instrumented & Tandem Downhole Deployment Valve (RIT-DDV). The purpose of this technology is to provide a temporary mechanical barrier to isolate and monitor the well during drilling operations in an environment where a full column of single-phase fluid cannot be maintained. The RIT-DDV is based on predominantly used downhole isolation valve (DIV) design and technology, which is a hydraulic flapper-type isolation device installed in the casing that seals the open hole during pipe tripping operations. The key features of the new RIT-DDV systems are dual flapper valves with three downhole pressure and temperature gauges to take measurements above, between, and below the flappers. The advantage of this configuration is that it enhances safety by enabling double-block-and-bleed system functionality, providing valve redundancy, and moreover allowing for continuous real-time monitoring of downhole well conditions. In addition, the RIT-DDV is designed to be reusable and can be tested upon installation and replaced if necessary. The RIT-DDV system enabled the operator to isolate and monitor the well while drilling through a depleted formation that prevented drilling with a full column of single-phase drilling fluid. The RIT-DDV was successfully trialed in western Kazakhstan and demonstrated the potential of this technology to enhance the safety of drilling heavily fractured carbonate formations with reservoir fluids containing hydrogen sulfide (H2S) / carbon dioxide (CO2) that are prone to total loss of circulation. The downhole pressure / temperature monitoring capabilities that the system provides within the casing string helped drill through the depleted fractured carbonate reservoir section without incurring non-productive time (NPT).

Quantification of the Loss Conduit Aperture During Lost Circulation in Fractured Carbonates

Fractured carbonate formations around the world are prone to lost circulation that not only affects the well construction process but creating a longtime effect on the wellbore integrity. Despite multiple attempts to cure them the success rate is usually low. This manuscript is aiming to provide a new vision on the reason of lost circulation across carbonates. To have better understanding of the complete losses across the fractured carbonates the series of studies were initiated. At first to understand the strength of the loss zone the fracture closing pressure was evaluated via study of the fluid level in the annulus and back-calculation of the drilling fluid density effect on it. Secondary, the rock properties across the loss circulation zones were studied by using the microresistivity images, dip data, and imaging of fluid-saturated porous media. At last, the trial tests with different treatment materials were performed to evaluate the effect of it on curing the losses. The results of the studies brought new information and explained some previous unknowns. The formation strength across lost circulation zone was measured and it was confirmed to remain constant despite other changes of the well conduction parameters. It was also confirmed that the carbonates are naturally highly fractured having over 900 fractures along the wellbore. The lost circulation zone was characterized, and it was confirmed that the losses were not related to the fractures but rather to the karst, dissolution and to mega-fractures. The size and dip of the fractures were identified, and it was proven the possibility to treat them with conventional materials. However, the size of identified mega-fractures and karst zones exceed the fractures by 100 times in true vertical depth, and in horizontal wells the difference is thousands times due to measured depth. This new information explains the previous unsuccessful attempts with the conventional lost circulation materials. Further based on the newly available information the mathematic description of the lost circulation zones was provided.