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Designing an acid-fracturing treatment is similar to designing a fracturing treatment with a propping agent. Williams, et al. presents a thorough explanation of the fundamentals concerning acid fracturing. The main difference between acid fracturing and proppant fracturing is the way fracture conductivity is created. In proppant fracturing, a propping agent is used to prop open the fracture after the treatment is completed. In acid fracturing, acid is used to "etch" channels in the rock that comprise the walls of the fracture.
Introduction The first hydraulic fracturing treatment was pumped in 1947 on a gas well operated by Pan American Petroleum Corp. in the Hugoton field. Kelpper Well No. 1, located in Grant County, Kansas, was a low-productivity well, even though it had been acidized. The well was chosen for the first hydraulic fracture stimulation treatment so that hydraulic fracturing could be compared directly with acidizing. Since that first treatment in 1947, hydraulic fracturing has become a common treatment for stimulating the productivity of oil and gas wells. Hydraulic fracturing is the process of pumping a fluid into a wellbore at an injection rate that is too great for the formation to accept in a radial flow pattern. As the resistance to flow in the formation increases, the pressure in the wellbore increases to a value that exceeds the breakdown pressure of the formation open to the wellbore. Once the formation "breaks down," a fracture is formed, and the injected fluid begins moving down the fracture. In most formations, a single, vertical fracture is created that propagates in two directions from the wellbore. These fracture "wings" are 180 apart and normally are assumed to be identical in shape and size at any point in time; however, in actual cases, the fracture wing dimensions may not be identical. In naturally fractured or cleated formations, it is possible that multiple fractures can be created and propagated during a hydraulic fracture treatment. Fluid that does not contain any propping agent (called the "pad") is injected to create a fracture that grows up, out, and down, and creates a fracture that is wide enough to accept a propping agent. The purpose of the propping agent is to prop open the fracture once the pumping operation ceases, the pressure in the fracture decreases, and the fracture closes.
Abstract Carbonate reservoir with low permeability is developed using fracturing technology to effectively stimulate the reservoir for a higher productivity. However, as the carbonate reservoir features extreme heterogeneity, not every fracture is created equally as is obtainable through acidizing fracturing or using proppant. The objective of this study is to help make the decision among acid fracturing or proppant fracturing, to achieve an economically and operationally feasible treatment through extensive research of field cases on critical factors and numerical simulation. An ideal fracturing method of a carbonate reservoir should yield a high conductive fracture with a potential for long production duration under depletion scenario within an economic limit. We first review real field cases to identify critical parameters affecting fracture geometry and productivity; then we validate and quantify their individual effectiveness using 3D synthetic models with numerical simulation. In the simulation work, both treatments are evaluated by simulating the production of the reservoir for 1,500 days post treatment. Results are compared for different reservoir parameter variations including porosity, permeability, temperature, depth and young's modulus. Simulation of the field is continued for 1,500 days after the treatment to see the effect of the production enhancement for both types of treatment. This is also done to include long term loss of fracture conductivity and its effect on the overall production. Fracture geometry from each case is also analyzed and compared to the base case scenario. Even though hydraulic fracturing and acidizing technologies have been the dominant stimulation methods in unconventional resources and many variants have been developed, there is still no well-defined guideline which can help an operator to select the stimulation techniques suitable for their particular fields. This paper focuses on factors influencing decision making on the stimulation of carbonate reservoirs based on reservoir characterization. The advantages and limitations of proppant fracturing and acid fracturing are compared based on field applications, which can enable an operator to make quick and informed decision on which stimulation should be used. Numerical effort confirmed and quantified our observations. Comparisons have been drawn on different types of proppant and acid treatments and their effectiveness on the target reservoir. The study eventually comes up with identifying main factors of reservoir for treatment selection and recommendations are presented from these findings that which type of reservoir is best suited for which type of treatment. The novelty of the study is summary of previous field cases with a clear guideline on making decision in carbonate reservoir stimulation. As a further extension to this study, PVT data can also be included to see the extent of reservoir fluid change effect on the outcome of the treatment.
Aljawad, Murtada Saleh (King Fahd University of Petroleum and Minerals) | Palharini Schwalbert, Mateus (Petrobras and Texas A&M University) | Zhu, Ding (Texas A&M University) | Hill, Alfred Daniel (Texas A&M University)
Summary The performance of acid‐fractured wells depends on the conductivity distribution along the acid‐penetration length, which is a function of reservoir properties, treatment design, and execution. The previously published models for fractured‐well performance assume constant fracture conductivity, which cannot be achieved in acid‐fracturing operations. This work proposes a design optimization method where acid‐fracture and reservoir models are integrated. Fracture‐conductivity distribution along the fracture surface is considered in the optimization process. In the new integrated model, the acid transport and reaction are joined to the fracture‐propagation and heat‐transfer models. The dissolution patterns along fracture surfaces are generated, and this is converted to a conductivity distribution. To predict the fractured‐well productivity, the reservoir model is built using the input reservoir properties, as well as the calculated acid‐fracture geometry and conductivity distribution. The acid‐fracturing parameters that lead to the optimal fracture productivity are determined with this integrated model. This method shows that there is an optimal productivity that can be obtained for a given acid treatment volume and reservoir properties. Design parameters such as flow rate, viscosity, acid concentration, acid treatment volume, and pad and overflush volumes can be selected to achieve optimal well performance. Reservoir permeability has an important impact on how acid‐fracture jobs should be designed. At low‐reservoir permeability, a more evenly distributed conductivity and a long acid‐penetration length are preferred. This can be accomplished by injecting a retarded acid system at moderate- to high-flow rate. However, excessive fracture‐height growth should be prevented by carefully designing the injection rate and viscosity. For high‐permeability reservoirs, higher conductivity along a shorter acid‐penetration length is targeted. This can be obtained by selecting a more‐reactive acid system such as straight acid and injecting at moderate rates, or by lowering the injection rate of retarded acids. A minimum amount of pad should be used in this case. Still, the flow rate should be highly sufficient to keep the fracture open during acid injection. We also show how acid concentration and fluid stages can be designed to optimize productivity and presents a procedure for selecting the acid treatment volume. A theoretical model, which integrates acid fracturing and reservoir flow, is used to implement the guidelines on optimizing acid‐fracture design parameters. In this paper we provide a scientific approach to determine acid treatment volume that yields optimal outcomes for acid fracturing.