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The evolution of horizontal drilling and multistage completions has changed matrix stimulation from the “more acid, better result” belief to effective lateral distribution and deeper penetration with less acid.
Large-gallon-per-foot-based treatment became impractical and unnecessary. The constant remains that drilling is blamed for causing formation damage; therefore, matrix stimulation is needed. Underbalanced coiled-tubing drilling aimed at changing this constant, however, has its own drawbacks.
For certain lithologies, acid tunneling can be an attractive alternative to rotary-bit drilling. Though the technology was proposed many years ago, its use has been limited. Hardware to control the drilling direction, fluids to dissolve the formation rock efficiently, and jetting dynamics to optimize the rate of penetration have been pushed forward progressively. The goal of stimulation while drilling finally may be achievable.
Of course, not all reservoir rock types are readily soluble in chemicals. Cracking the reservoir rock will continue to be the preferred process for sandstone. The first mechanism we think of for rock cracking is hydraulics. Unless full-fledged hydraulic fracturing is required, using high-energy pulses induced by propellant or other chemical reactions can produce sufficient pressure to break the rock past the near-wellbore formation damage. Perfecting the control and job design can lead to efficiency and broader implementation.
One question we often ask in the matrix-stimulation domain is “what’s new?” It almost seems that the products and technologies of yesterday have been brought out again and again with minor twists. Indeed, it is amazing how many “new technologies” actually have been around for decades.
While most of the industry is busy handling daily operations and logistics, it is encouraging to see universities, government-supported research and development institutes, and even some large service and operating companies studying the science behind the products and technologies. Advanced multiscale, multiphysics mathematical models are used to optimize particle bridging for diverting efficiency; large-scale experimental setups are used to gain insights into differential etching patterns during acid fracturing caused by viscous fingering and heterogeneous reactions, perforation penetration in realistic geometry and stresses, and other phenomena that could not be observed by small-scale laboratory testing. These studies help us appreciate the ingenuity of our predecessors and help us fit the technologies to the right applications better.
Recommended additional reading at OnePetro: www.onepetro.org.
SPE 185344 Application of Closed-Fracture Acidizing for Stimulation of Tight Carbonate Reservoir in Mumbai Offshore by Dilip Kumar Sarma, Oil and Natural Gas Corporation, et al.
SPE 187019 Large-Scale Visual Experiment and Numerical Simulation of Acid Fingering During Carbonate Acid Fracturing by Xiaogang Li, Southwest Petroleum University, et al.
SPE 189546 Influence of Transport Conditions on Optimal Injection Rate for Acid Jetting in Carbonate Reservoirs by Dmitry Ridner, Texas A&M University, et al.
Carbonate rock holds 60% of the global oil and gas reserves, but they are becoming more and more expensive and difficult to develop. With large reservoirs maturing, operators are forced to explore and produce from deeper resources, which are tight, highly stressed, and under high temperature. In today’s economic environment of USD 50/bbl, the cost of extracting hydrocarbon from these reservoirs needs to be scrutinized to maximize profitability. This means increasing drainage of wells using effective stimulation and optimizing production profile along the well.
Generically, carbonate matrix stimulation means pumping acids, retarded or unretarded with various functional additives, through coiled tubing or by bullheading, followed with diverting agents. Treatment volume and injection rate are based on rules of thumb formed from experience accumulated in the industry and based on laboratory studies with cores under achievable experimental conditions. Success is evaluated from the pressure and temperature response during treatments or incremental production increase after stimulation. There has been ongoing development to add science and engineering to the art of matrix stimulation so that the fluid volume can be reduced, the placement can be more controlled, and the result can be measured more directly and reliably. Adapting multiple operations in a single trip, by combining mechanical tactics such as jetting and chemical tactics such as energized or in-situ-generated acid, further allows time and cost saving while minimizing risks and enhancing well productivity. Measuring real wormhole penetration in the reservoir will help complete the loop of design, execution, and evaluation. It adds tremendous value for engineers to optimize the matrix treatment. Alternative physics and chemistry are on the horizon as well. Temperature-induced fracturing is one idea to increase well and reservoir connectivity, although the ability to control the depth of fracturing still needs to be worked out. It might help operators tailor stimulation in particular reservoir and geological properties for bypassing near-wellbore damage.
Tight formations are candidates for hydraulic fracturing as the default. However, the solubility of carbonate by various chemicals provides opportunities to extend the well drainage radius effectively without the intensive equipment, material, and infrastructure demand of hydraulic fracturing. The industry has developed significant portfolios of technologies for stimulating carbonate reservoirs, covering intervention tools, pumping processes, chemicals, and diagnostic mechanisms. Standing alone, no technology can deliver productivity optimization and maximized cost-effectiveness. Integrating technology-provider ingenuity with operator knowledge in applicability will be key to using these technologies to make matrix stimulation more effective in delivering hydrocarbons from the increasingly difficult-to-tap resources.
Recommended additional reading at OnePetro: www.onepetro.org.
SPE 181845 Far-Field Diversion in Hydraulic Fracturing and Acid Fracturing: Using Solid Particulates To Improve Stimulation Efficiency by Vanessa Williams, Baker Hughes, et al.
SPE 183465 A Novel Approach for Stimulation of a Heterogeneous Thin-Layered Reservoir in an Offshore Field, Abu Dhabi by S.F. Nofal, Abu Dhabi Marine Operating Company, et al.
SPE 181823 A New Acid-Fracturing-Fluid System for High-Temperature Deep-Well Carbonate Reservoir by Ying Gao, China National Petroleum Corporation, et al.
The economic condition of the oil and gas industry is tighter than ever. Optimizing well productivity to maintain cost effectiveness is on every operator’s mind. Enhancing well productivity requires more-detailed analysis, more measurements, and even-more-advanced products and technologies, which all cost more money. Doing more and reducing cost can be a dilemma. This is the time when properly choosing sound technologies is critical to improve efficiency and increase success rates. Several well-stimulation products and techniques have been seen to benefit well productivity from recent field trials and implementations in carbonate reservoirs, including simpler acid fluid systems, integrated work flows, and coiled-tubing bottomhole assemblies. Emulsified acid has been a preferred carbonate-stimulation fluid for 30 years. Clearly, mass transfer can be retarded significantly by the emulsion form for deeper live-acid penetration in matrix- and fracture-acidizing applications. The quality of the emulsified acid is sensitive to how it is mixed. In the laboratory, slowly adding acid into oil, mostly diesel, containing surfactant while blending is required to achieve good emulsion stability. Field practice is another story. The large amount of emulsified acid is normally batch-mixed in the service company yard or at the wellsite by circulation pumps and tanks. The mixing energy used in the laboratory is difficult to match in the field. Consequently, the consistency of the final emulsified acid pumped can vary from well to well. Recently, single-phased-acid systems have been developed to simplify the fluid preparation and to make the product performance more robust. Various single-phased acids are provided by service companies or even operators themselves using different formulations to achieve the effect of retardation. Another important technology for successful acidizing, especially in horizontal wells or highly heterogeneous reservoirs, is diversion. Many diverters have been used. Evaluation of diversion during the treatment remains rudimentary. Pressure humps are used to infer diversion without good confirmation of how effective the diversion actually is. More-accurate information about fluid rheological and friction characteristics is needed to evaluate the downhole-pressure behavior and, hence, quantified diversion results. The term “diversion” is often used interchangeably with “placement.” Proper placement can be achieved by means other than diverting materials. Jetting is believed to place acid effectively in matrix stimulation without chemical diverters. Each technology costs additional money at the early stage. When they are routinely used, fully integrated in work flows, and eventually linked with automation, true cost effectiveness of the stimulation treatments can be realized. The investment made today, therefore, will bear fruits in the future.
The fourth industrial revolution is taking the oil and gas business by storm. Many companies have increased resources for developing and using big-data analytics and machine learning to catch the wind that could bring the fragrance of the new economy. In a conversation with management of a major service company, I learned that the downturn made them want to shift from a position as a capital-intensive stimulation service provider to that of a higher-profit-margin data-service provider. Though no one sees the actual physical oilfield services—such as pumping, measurement, and chemicals—as in decline, technology development in these areas may take a back seat to artificial intelligence. That said, technological innovation is not a privilege exclusive to scientists and high spenders in research and development but is often the work of good engineers. Integrating and applying existing (or even old) technologies to do the things we know how to do better is our operational model in many ways in this low- price environment.
If we do not feed the right data to the machine-learning process, we are not doing justice to the smart machines we build. An example of the right data is downhole pressure, particularly during acid fracturing. We do not know whether we routinely conduct high-rate matrix acidizing or acid fracturing unless we have reliable downhole pressure data. Otherwise, we may dissolve a large rock mass in an undesirable manner and location, failing to optimize stimulation design.
The other technology area to improve in stimulation is the measure of diversion. We have been using particulate diverters for ages. Now, we need such products to function under more-challenging conditions. Instead of diverting from matrix, we want them to divert from fractures. Advanced experimental equipment and testing procedures are necessary to gain better quantitative information to determine what size diverter pill is appropriate to build sufficient pressure for inducing new fractures.
Matrix-stimulation techniques predominantly rely on chemicals to attack the rock and mechanical tools to place the chemicals. The aspect of physics-enhanced chemical reactions has not caught many eyes, though it has been around for a while. Some oil-production increases have been reported after earthquakes. Geophysicists provided plausible reasons, but petroleum engineers only explored such applications superficially. Not every well can be hydraulically fractured nor every rock dissolved by acids. We need all the help we can get in this kind of situation. Combining acoustic waves with chemicals for stimulation definitely should be studied more. It is encouraging to see people persevere in pursuing continuous improvements in stimulation technologies by going back to the shelf.
Recommended additional reading at OnePetro: www.onepetro.org.
SPE 191981 Introduction of Real-Time Flow Measurements Opens New Paths To Overcome Challenges Encountered During the Acid Stimulation of Extended-Reach Wells by Laurie Duthie, Saudi Aramco, et al.
SPE 192069 Development and Application of Key Equipment of CO2 Waterless Fracturing by Lichen Zheng, PetroChina, et al.
SPE 193723 Effective Matrix Acidizing Based in Chelating Agents: A Case Study in Romanian Heavy-Oil Reservoirs by Emil Panait, OMV Petrom, et al.
Summary Fracture acidizing has been a dominant practice in the industry to enhance well productivity in low-permeability carbonate reservoirs. Many acid systems have been developed to improve this stimulation process. The most desirable characteristics for an acid system to be suitable for fracture acidizing are leakoff control and retarded reaction rate. These characteristics are required for deep acid penetration, so that when the fracture closes, long flow channels are etched on the fracture surfaces. Leakoff control can be achieved by pumping a pad containing a viscosifying agent or solid bridging agents to plug wormholes generated by acid dissolution. Reaction retardation is attempted usually by lowering the effective diffusivity of the hydrogen ion. It is well known that during an acid-fracturing operation, the overall reaction rate of hydrochloric acid (HCl) with limestone is mass-transfer-limited. Designing the treatment requires knowing the effective diffusivity of the hydrogen ion in the acid system, which, to the best of the authors' knowledge, has not been determined before. Because of their combined leakoff-control and retardation capabilities, surfactant-based acids have been used in acid-fracturing treatments. Because more carbonate reservoirs are treated by use of this acid system, it is important to obtain the effective diffusivity of H. The rotating-disk device has been used to investigate the reaction kinetics between a reactive solution and carbonate rocks because the thickness of the boundary layer is uniform throughout the disk surface. This paper discusses the reaction-rate data generated recently for surfactant-based acid by use of a rotating-disk apparatus and presents the methodology used to determine the effective diffusivity from the measurements. The results obtained indicated that the viscoelastic surfactant examined (carboxybetaine-type) reduced the dissolution rate of calcite with HCl acid. The surfactant reduced the diffusion coefficient for H. The effect of temperature on the diffusion coefficient did not follow the Arrhenius law.