Predicting Productivity of Multiple-Fractured Horizontal Gas Wells

In low-permeability gas reservoirs, horizontal wells have been used to increase the reservoir contact area, and hydraulic fracturing has been further extending the contact between wellbores and reservoirs. This paper presents an approach to evaluate horizontal well performance for fractured or unfractured gas wells, and a sensitivity study of gas well performance in low permeability formation. A newlydeveloped Distributed Volumetric Sources (DVS) method was used to calculate dimensionless productivity index for a defined source in a box-shaped domain. The unique features of the DVS method are that method can be applied to both transient flow and pseudo-steady state flow with a smooth transition between the boundary conditions. By describing the geometric dimensions of the sources, productivity index for a horizontal gas well or a longitudinal fracture along a horizontal well is readily calculated. For transverse fractures with infinite conductivity or uniform flux, the superposition principle is used for multiple sources in the system. Wellbore frictional pressure drop is coupled with the inflow calculation from fractures. The study compares the productivity of horizontal wells versus horizontal well(s) with hydraulic fractures. The effects of parameters such as reservoir permeability and anisotropy ratio, horizontal well spacing and placement, and orientation of fractures along a horizontal wellbore are also presented in the paper. The objective is to optimize well design based on the maximum productivity and economic benefit. The results of the study show that the performance of horizontal wells can be very sensitive to the vertical permeability of the formation. In low vertical-permeability formation (high anisotropic ratio), fractures improve well performance significantly. In large drainage area, multiple-well placement, even with shorter wellbore length, increases productivity, compared with singlewell placement. The study provides insight on optimization of production performance in low-permeability gas reservoirs. Introduction Development of low permeability gas reservoirs, conventional or unconventional, is one of the solutions to today’s energy supply and demanding problem. In low-permeability gas reservoirs, creating flow path is critical, and horizontal wells have been extensively used to increase the reservoir contact area. Hydraulic fracturing can further expand the contact between wellbores and formations. For horizontal wells, with or without hydraulic fracturing, well performance becomes very sensitive to permeability and anisotropic ratio when the reservoir permeability is low. If the vertical permeability is the formation is extreme low (high anisotropic ratio) then the benefit of horizontal wells starts diminishing. In such a case, hydraulic fracturing provides another option to increase well productivity. When hydraulically fracturing a horizontal well, created fractures can be single longitudinal, multiple longitudinal, single transverse, or multiple transverse. The orientation and placement of fractures along a horizontal well greatly affect the performance of the well. Depending on the formation condition and fracturing design, fracture may result in unflavored productivity in some cases, and this has been evidenced in the field. Predicting well performance for fractured and non-fractured horizontal wells can help to bestproduce from low permeability gas formations. In the past, analytical models have been used to calculate productivity of horizontal well. The models are developed under assumptions about the boundary conditions. Steadystate models assumed a constant pressure at the drainage boundary, pseudo-steady-state models assumed no flow crossing the boundary with either constant pressure gradient or constant flow rate, and transient flow models uses an infinite acting drainage domain. For low permeability formation, flow condition is more likely in the transient period relatively longer before reaches the pseudo-steady state flow condition compared with normal-permeability formations, and the transition from transient flow to pseudo-steady state flow is problematic. There are also analytical models for fracture performance of horizontal wells. Recently, Economides modified the fracture performance models for vertical wells, and used a pseudo-skin factor to account for the converge effect for transverse fractures along a horizontal well. For multiple fractures, drainage area can be divided into sub-area, and no-flow assumption is used between sub-areas. In this paper, we present a different approach to calculate well productivity for horizontal gas wells with or without fractures. Fractures can be longitudinal or transverse, single or multiple, with infinite conductivity or uniform influx. The well performance is represented by a dimensionless SPE 106280 Predicting Productivity of Multiple-Fractured Horizontal Gas Wells D. Zhu, F. Magalhaes, and P.P. Valkó, Texas A&M U.