Mohamed Y. Soliman

Fracturing aspects of horizontal wells

This paper discusses the main reservoir engineering and fracture mechanics aspects of fracturing horizontal wells. The paper discusses fracture orientation with respect to a horizontal wellbore, locating a horizontal well to optimize fracture height, determining the optimum number of fractures intercepting a horizontal well, and the mechanism of fluid flow into a fractured horizontal well.

Hydraulic Fracturing Design and Optimization: A Modification to Zipper Frac

In recent years, new fracturing designs and techniques have been developed to enhance production of trapped hydrocarbons. The new techniques focus on reducing stress contrast during fracture propagation while enhancing far field complexity and maximizing the stimulated reservoir volume. Zipper frac is one of these techniques, which involves simultaneous stimulation of two parallel horizontal wells from toe to heel. In this technique, created fractures in each cluster propagate toward each other so that the induced stresses near the tips force fracture propagation to a direction perpendicular to the main fracture. The effectiveness of zipper frac has been approved by the industry; however, the treatment’s optimization is still under discussion. In this paper, we present a new design to optimize fracturing of two laterals from both rock mechanic and fluid production aspects. The new design is a modification to zipper frac, where fractures are initiated in a staggered pattern. The effect of well spacing on the changes in normal stress has been evaluated analytically to optimize the design. Results demonstrate that the modified zipper frac improves the performance of fracturing treatment when compared to the original zipper frac by means of increasing contact area and eventually enhancing fluid production. Introduction Hydraulic fracturing is a stimulation technique used to extract trapped hydrocarbon. Fracturing vertical wells was used for variety of reservoir conditions varying from tight gas formations to high permeability formations implementing the FracPac applications. Fracturing horizontal wells started in the late 80’s for stimulation of tight gas formation. The use of fracturing horizontal wells proved to a key technology in the development of unconventional reservoirs. The technique has been widely used with the development of Barnett shale in the late 90s (Navigant Consulting, 2008). While the existence of natural fractures in shale oil and gas plays make them good candidates for hydraulic fracturing, the key in a successful treatment is creating a complex network that connects created hydraulic fractures with pre-existing natural fractures. This network of fractures, which consist of hydraulic fractures, primary and secondary natural fractures, are highly desired in low permeability reservoirs where higher conductive connectivity can be achieved as opposed to connectivity created by planar fractures (Soliman et al. 2010). Numerical simulations (Mayerhofer et al. (2008); Nagel and Sanchez-Nagel (2011); Warpinski et al. (2009); Cipolla et al. (2009) show that creating an interconnected network of fractures in nano-permeable reservoirs is a major factor in economic production. Various methods have been applied to create this complex network and ultimately maximize the total Stimulated Reservoir Volume (SRV). Creating secondary fractures is a vital occurrence in increasing the reservoir contact. Secondary fractures can be created by multistage fracturing along a horizontal wellbore in a naturally fractured reservoir. Different design parameters including the number of perforation clusters per stage, the spacing between stages, the length of the horizontal well, the sequence of fracturing operations, and the type and quantity of proppant should be optimized to create secondary fractures and a complex network of fractures (Mayerhofer et al. 2010). Among these parameters, spacing between perforation clusters as well as fracturing stages play major roles in fracture propagation and geometry. As noted by Soliman et al. (2008), the spacing between fractures is limited by the stress perturbation caused by the opening of propped fractures. However, fracturing designs can be optimized if the original stress anisotropy is known and the stress perturbation can be predicted (Soliman et al. 2010). Recent advances in fracturing design (East et al. 2010; Cipolla et al. 2010; Roussel and Sharma 2011; Waters et al. 2009) offer techniques for creating far field fracture complexity to enhance the SRV. Zipper frac is one of these techniques in which two horizontal wellbores are fractured simultaneously to maximize stress perturbation near the tips of each fracture. The

Rock mechanics and stimulation aspects of horizontal wells

In certain reservoir conditions, horizontal wells can offer significant production improvement over vertical wells; however, fracturing is often required to maximize the return on investment for these wells. Since its introduction in the late 1980s, the practice of fracturing horizontal wells has become a viable completion option. This is especially true in the case of tight gas formations. This paper reviews best practices in the fracturing of horizontal wells and includes a discussion on the rock mechanics, operational strategies, and the reservoir engineering aspects of fracturing horizontal wells. The rock mechanics discussion reviews the theoretical and experimental work and creation of: (1) transverse and longitudinal fractures, (2) multiple fractures, and (3) fracture reorientation among others factors that are associated with creation of a fractured horizontal well. Stability of the horizontal well as it relates to stimulation is also discussed. The reservoir engineering portion discusses the production performance and testing aspects of a fractured horizontal well. Emphasis is given to fracturing tight gas formations since this area is the one in which this technique is considered to be the most effective. The performance of a longitudinal fracture is examined and compared to a fractured vertical well and to the more popular transverse-fractured horizontal well. Because performance of a longitudinal fracture is similar to that of a fracture in a vertical well, the existing solutions for fractured vertical wells may be applied to longitudinal fractures. This approximation is valid for moderate to high dimensionless fracture conductivity. In the case of transverse fractures, the outer fractures outperform the inner fractures. However, for most cases, more than two fractures are necessary to efficiently produce the reservoir. Operational aspects of fracturing horizontal wells for both transverse and longitudinal fractures are discussed, and advantages and disadvantages of each type are outlined. Examples and case histories are given. The paper also presents guidelines for stimulation of a horizontal well and includes both propped-and acidized fracturing as well as matrix acidizing.

Evaluate EOR Potential in Fractured Shale Oil Reservoirs by Cyclic Gas Injection

The current technique to produce shale oil is to use horizontal wells with multi-stage stimulation. However, the primary oil recovery factor is only a few percent. The low recovery and the abundance of shale reservoirs provide a huge potential for enhanced oil recovery. Well productivity in shale oil and gas reservoirs primarily depends upon the size of fracture network and the stimulated reservoir volume (SRV) which provides highly conductive conduits to communicate the matrix with the wellbore. The natural fracture complexity is critical to the well production performance and it also provides an avenue for injected fluids to displace the oils. However, the disadvantage of flooding in fractured reservoirs is that the injected fluids may break through to production wells via the fracture network. Therefore, a preferred method is to use cyclic gas injection to overcome this problem. In this paper, we use a numerical simulation approach to evaluate the EOR potential in fractured shale oil reservoirs by cyclic gas injection. Simulation results indicate that the stimulated fracture network contributes significantly to the well productivity via its large contact volume with the matrix, which prominently enhances the macroscopic sweep efficiency in secondary cyclic gas injection. In our previous simulation work, the EOR potential was evaluated from planar traverse fractures. In this paper, we examine the EOR potential by including the effect of fracture networks. Therefore, a higher oil recovery potential is demonstrated. The impacts of fracture spacing density and stress dependent fracture conductivity on the ultimate oil recovery are also investigated. In a case where the fracture network spacing is 100 ft and the fracture network is 100% stimulated, it can achieve more than 60% of incremental oil recovery. The results presented in this paper demonstrate an EOR potential by cyclic gas injection in fractured shale oil reservoirs.

Design Optimization of Horizontal Wells With Multiple Hydraulic Fractures in the Bakken Shale

Hydraulic fracturing is currently the completion method of choice in most tight reservoirs; however, the ultimate performance of fractured wells is severely affected by the interfering effects inside the fracture and interfractures. Previous simulation studies investigated the effects of well spacing and fracture length on well productivity in low-permeability oil and gas reservoirs. It was shown that the most important parameters for determining the optimum fracture length are the formation permeability and the stimulated reservoir volume (SRV). Although a number of studies have examined the performance of horizontal fractured wells and the fracture geometry effect, fracture spacing and intersecting angles in vertical and horizontal wells should be further investigated. This study presents the results of a tight oil reservoir analogy. Reservoir parameters considered include local rock stresses, rock compressibility, absolute and relative permeability, and porosity. The well-completion parameters included fracture length, height, width, conductivity, number and spacing between fractures, fracture intersecting angle, and cased- vs. openhole completion. Fracture modeling considered rigorous description of the hydraulic fracture properties and finite difference reservoir modeling. Economically attractive reserves recovery was modeled through multiple fracture placements in a 10,000-ft horizontal well. Numerical simulation showed that oil recovery increased between 8 to 15%, while net present value (NPV) increased 8 to 24%, as the number of fractures increased. Based on the critical assumptions in the study (permeability, natural fracture distribution, and stress orientation), an optimum number of fractures was identified. Openhole completions provided better performance in most cases, and recovery was greater for a higher number of contributing perpendicular vs. longitudinal fractures. The results of the study hopefully can be used to improve the understanding of the role of fracture geometry, spacing, and open/cased-hole completion strategy to enhance an operator’s optimum completion design.

Approximate solutions for flow of oil heated using microwaves

Abstract This paper describes development of a model of oil flow when microwave technology is used to heat the reservoir. Since this model presents a highly complex problem, the solutions are given with two levels of simplification. First, the problem is solved numerically. Further simplification of the problem yields a form that can be solved analytically. Procedure to use the simplified analytical solution to gain insight into the more completed solution has been presented. The numerical solution is presented for cases in which heat loss into the adjacent strata has been considered as well as for cases in which the heat loss has been ignored.

Fracturing Horizontal Wells in Gas Reservoirs

The fracturing of horizontal wells has recently gained wide acceptance as a viable completion option to maximize the return on investment. This is especially true in the case of tight gas formations. Horizontal wells have unique rock mechanics and operational aspects that affect fracture creation and propagation and control fracture orientation with respect to the horizontal well. The fracture orientation greatly affects the productivity and well testing aspects of the fractured horizontal wells. Depending on stress orientation relative to the wellbore, the fractures may be transverse or longitudinal. This paper presents a model for fractured horizontal wells operating under constant pressure conditions. This condition is most suitable for producing tight gas reservoirs. The model considers the presence of either transverse or longitudinal fractures. In this paper, we examine the factors involved in determining the optimum number of transverse fractures for both finite and infinite reservoirs. For a group of transverse fractures, the rate distribution for each fracture is presented and analyzed. The effect of uneven fracture length is briefly presented. The performance of a longitudinal fracture is examined and compared to a fractured vertical well and to a transverse-fractured horizontal well. A comparison of longitudinal versus transverse fractures from reservoir and operational points-of-view is presented. Also included is a short discussion of field examples. Because performance of a highly conductive longitudinal fracture is almost identical to that of a fractured vertical well, the existing solutions for fractured vertical wells may be applied to longitudinal fractures with a high degree of confidence. This approximation is valid for moderate to high dimensionless conductivity. In the case of transverse fractures, the outer fractures outperform the inner fractures. However, in most practical cases, more than two fractures are necessary to efficiently produce the reservoir. A simplified economic analysis supports this conclusion.

Impact of Fracturing and Fracturing Techniques on Productivity of Unconventional Formations

Unlocking the potential of unconventional gas reservoirs can change the balance and future of the oil industry. Unconventional gas reservoirs can be tight-gas, coalbed methane (CBM), or shale reservoirs. Economic production of any of these three types requires the creation of multiple fractures from a long horizontal well. Fracturing horizontal wells presents several challenges regarding the rock mechanics, change of stresses around the created fractures, and fluid flow. New and reinterpreted laboratory experiments have shed new light on fracturing a horizontal well and the effect of how the well is completed on the fracturing process. The results could explain the presence of multiple fractures at the wellbore. These geomechanical issues could influence the fracturing process, especially in naturally fractured formations. This paper investigates the effect of various fracturing scenarios on the stress distribution around the fractures. Optimization of the number of fractures is also investigated from both fluid-flow and geomechanical points of view. Special attention is given to shale formations for two reasons—because of the great potential of shale formations, and because of the special characteristics that makes shale unique and challenging. Shale formations have ultra-low permeability that can be in the nanodarcy range. Shale formations are naturally fractured, and, depending on the carbon content, can have a significant amount of adsorbed gas. This paper also investigates the effect of gas adsorption on productivity. Field examples are presented.

Evaluation and completion procedure for produced brine and waste water disposal wells

Abstract Waste water disposal is one of the many environmental issues becoming increasingly difficult to deal with. A single disposal well may not be sufficient to handle the large volume of waste water generated by many industrial plants. Increasing the injectivity of a well by hydraulic fracturing is a viable solution to this problem. This paper discusses the application of a numerical water injection simulator to evaluate a low-permeability sandstone reservoir for waste water disposal. The injection model is used to study the effects and feasibility of hydraulically fracturing the reservoir to improve injectivity. Also discussed are the field implementation of the hydraulic fracturing treatment, testing of the well to evaluate the treatment, and a comparison of observed and simulated injectivity.

Numerical Simulation Of Thermal Recovery Processes

This study presents the development and application of an In-situ combustion model designed to simulate a laboratory combustion tube. This model is a first step to a comprehensive field model. It considers both mass and heat balance equations. Combustion is assumed to occur from direct burning of oil. Except for this assumption, the simulator rigorously considers the flow of different fluid phases through a combustion tube. To expand the mass and heat balance equations, the authors use a fully implicit finite difference scheme. The resulting system of equations is solved simultaneously. The model calculates data on pressure, temperature and saturation distribution inside a combustion tube. These data are compared to published laboratory combustion tube data as well as other existing numerical simulators. The model also simulates hot water flooding and steam injection. 31 refs.