Optimized Oilwell Fracturing of Moderate-Permeability Reservoirs

Abstract

SPE Member Abstract This paper presents results from an analysis of the effect of fracturing on moderate permeability oil reservoirs under primary depletion and secondary recovery processes. This analysis indicated that the optimum fracture half length in wells having a reservoir permeability of 1-10 md is relatively short and highly conductive. Both simulated and field production data are used to support this conclusion. production data are used to support this conclusion Introduction Historically, fracture stimulations have been used to overcome the detrimental effects of wellbore damage. These "conventional" treatments were generally small and of limited capacity and lateral extent. In recent years, increased industry activity and interest in domestic production has led to improvements in fracturing equipment and materials, a greater understanding of fracture mechanics and ultimately, to the development of massive hydraulic fracturing (MHF). This technology has been used to create long finite conductivity fractures, which alter the flow geometry around the wellbore and provide increased surface area to flow, making many "tight" gas reservoirs commercially attractive. However, in moderate permeability oil reservoirs, the creation of short highly conductive fractures, as opposed to massive hydraulic fractures, has been found to be economically attractive. It is the purpose of this paper to show a process by which the optimum fracture length and fracture conductivity can be determined for wells in moderate permeability oil reservoirs under primary depletion permeability oil reservoirs under primary depletion and secondary recovery processes. The optimum fracture parameters determined in this manner are compared to "conventional" fracture treatments and used to modify fracture treatment designs. Both simulated data and field production data are presented to show the economic benefits derived from the creation of short, highly conductive fractures in moderate permeability oil wells. permeability oil wells. OPTIMIZATION PROCEDURE The optimization of oil well fracturing requires that the performance and cost of a given fracture configuration can be predicted. Reservoir and fracture design models, respectively, were used for this purpose. Results of these simulations were then combined in an economic model to optimize oil well fracturing. Oil well fracturing optimization requires that reservoir simulations be performed to determine the reservoir response to a given fracture configuration. Either steady state or reservoir models can be used to simulate the performance of fractured wells. However, reservoir models were used in this analysis to allow the economic benefits of unsteady state flow, rate acceleration, and the time value of money to be considered in the economic decision. In addition, use of a reservoir model allowed the effects of well spacing, fracture conductivity, length, and orientation to be evaluated as well. Two models were used in this analysis. A three-phase, two-dimensional, semi-implicit type model was used to predict the performance of a fracture in a reservoir under primary performance of a fracture in a reservoir under primary depletion performance. Areal simulations with fractures of half length xf, and conductivity, kfw, were performed in this analysis of the effects of fracturing on primary depletion performance. A three-phase two-dimensional "black oil" model with variable bubble point option was used for the analysis of the effects of fracturing on secondary recovery processes. processes.