Summary This paper presents experimental results related to hydraulic fracturing of a horizontal well, specifically the nonplanar fracture geometries resulting from fracture initiation and propagation. Experiments were designed to investigate nonplanar fracture geometries which were categorized as: multiple parallel fractures, T-shaped fractures, and reoriented fractures. This paper discusses how these nonplanar fractures can be responsible for premature screen out and excessive treatment pressure when a horizontal well is hydraulically fractured. Reasons for unsuccessful hydraulic fracturing treatments of a horizontal well are presented and recommendations to ensure clear communication channel between the wellbore and the fracture are given. Fracture initiation pressure is a function of the wellbore orientation relative to the maximum horizontal stress. It is maximum when the horizontal well is drilled in the direction of the minimum horizontal stress. Extension pressure, however, shows little variation with respect to the wellbore direction. Experimental data shows that a relationship between breakdown pressure and wellbore orientation exists and can be used for wellbore stability problems. Microfrac analysis has been applied on the experimental data and showed that care must be exercised to determine closure pressure from a microfrac test performed on a horizontal well. A new phenomenon, "Relief in Pressure (RIP)", is presented to help determine the fracture azimuth relative to the wellbore from observing the pressure behavior after breakdown. The results of these openhole experiments can also be used for perforation design in cased holes to obtain a successful hydraulic fracturing treatment. Optimum perforated interval length and perforation phasing should follow the fracture azimuth at the wellbore as ft initiates from a given horizontal well. Introduction Horizontal wells have been drilled to increase the effective drainage area as compared to a vertical well, which can penetrate a smaller portion of the same reservoir. A horizontal well is fractured to improve productivity. Fracturing a horizontal well has sometimes been a dilemma during the last few years with occurrence of premature screenouts and high treatment pressures. This work identifies some of the problems, and suggests some solutions. In most geological formations, the orientation angle of a horizontal well from the maximum horizontal stress plays a crucial role in achieving a successful stimulation treatment. The following three mechanisms related to wellbore orientation relative to the maximum horizontal stress (orientation angle), need to be addressed.Fracture-wellbore communication area. Two extreme cases, longitudinal and orthogonal fractures, provide maximum and minimum communication area between the wellbore and propagating fractures.Fracture geometry near the wellbore. Important factor that may cause early screenouts. Several different fracture geometries can result when a horizontal well is fractured, among which are multiple fractures, T-shaped fractures, and complex fractures.Fracture tortuosity near the wellbore.
Laboratory hydraulic fracturing experiments on unconfined coal blocks were conducted to simulate fracture stimulation of shallow coal seams. Fractures were initiated by injecting a gelled water into an openhole section of a wellbore. Multiple fractures appeared at the sample periphery during injection period. This paper describes observations made during laboratory experiments, and discusses implications of the experimental results to field-scale treatments. The results have application in modifying fracture width calculations for shallow coal seams. When a shallow coal seam is hydraulically fractured, the created fracture width may be developed as a result of inflating some cleats and closure of surrounding cleats. Stress-strain curves for coal samples under uniaxial compression showed nonlinear, stress-dependent Young's modulus. The traditional approach for the fracture width calculation based on a constant Young's modulus is not valid in shallow coal seam fracturing. Therefore, an approach that accounts for nonlinear compaction using stress-dependent modulus in calculating fracture width is described.
The partial differential equation describing the flow of fluid in a porous media was derived by combining the continuity equation (material balance), Darcy's equation of flow and the equation of state. The partial differential equation was linearized by neglecting high order gradient terms such as (∂2p/∂r2). The resulting partial differential equation is the well known parabolic diffusivity equation. This diffusivity equation which incorporates the effect of welibore storage and skin factor on the inner boundary condition has been solved1 and the solution is used to analyze the pressure drawdown. The pressure buildup testing theory was derived from the pressure drawdown theory described above. By definition, a pressure buildup test consists of two parts; the drawdown (flow) and shut-in periods. Traditionally, the equations defining a buildup test were derived using the diffusivity equation utilizing the principle of superposition. Thus it was implicitly assumed that the equations governing fluid flow were linear and that constancy of inner boundary conditions prevail throughout the drawdown and shut-in periods. Although these conditions are usually met, certain conditions may exist that prevent application of superposition. For example, requirement of linear partial differential equation which is essential for the application of superposition, is not fulfilled when the turbulent condition prevails during drawdown period. During the shut-in period however, the fluid velocity inside the reservoir diminishes quickly and turbulent effect ceases to exist. This turbulent flow is known to occur in high permeability gas reservoirs producing at high rates. The turbulent flow is usually accounted for by modifying the fluid flow equation for laminar flow by adding turbulent flow term.
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