Summary

This paper describes the theory and application of a new matrix stimulation diversion technique, maximized pressure differential and injection rates (MAPDIR), that uses injection rate as the key parameter to obtain sustained, planned bottomhole differential pressure, ?p, levels. The technique, applicable wherever Darcy's law is valid (in sandstones and in unfractured or microfractured carbonatic reservoirs), favorably replaces diverting agents and extends to long intervals (> 100 ft) the main advantage provided by mechanical zone isolation for "selective stimulation," which is maximized ?p. The use of slightly viscosified stimulation fluids can further extend the range of application and improve the economics by reducing both the amount of hydraulic power and the fluid volumes required for stimulation. A multilayered reservoir case illustrates the application of the MAPDIR technique. A new log-log plot of skin evolution vs. injected-acid volume shows significant damage removal trends and provides important information for matrix stimulation treatment optimization. Field experience includes the successful treatment of horizontal pay intervals of up to 460 ft.

Because the real objective of diversion techniques is the effective removal of damage from the whole exposed pay, the meaning of "diversion" in this paper is not narrow (i.e., applicable only to diverting agents and ball sealers) but includes "placement" and "coverage" techniques, such as mechanical isolation, viscosified acids, and MAPDIR.

Introduction

One of the most important factors affecting the success or failure of matrix stimulation treatments is the correct downhole placement of fluids for optimal zone coverage. When injected, these fluids naturally tend to follow the path of least resistance - i.e., into the higher-permeability and/or least-damaged zones. Because damage must be removed from the entire producing interval, effective diversion techniques must be used. To this end, numerous methods and techniques have been investigated and discussed in the literature. Mechanical methods (zone isolation packers, packers and bridge plugs, and straddle packers) for selective injection are considered the most effective form of diversion. However, they are cumbersome, expensive, and time-consuming and generally require a rig on site. Ball sealers offer a cheaper and more practical approach, but their effectiveness depends on many parameters, including length of perforated intervals, shot density, roundness and smoothness of perforation holes, injection rate, differential pressure across the perforations, and quality of the cement bond. Furthermore, mechanical methods cannot be applied in gravel-packed and openhole completions. The use of diverting agents has been seen as a simpler way to overcome the fluids' natural tendency to invade the most-permeable/least-damaged zones. At least in principle, particulate diverters enable the flow to be equalized by depositing a cake on the reservoir rock face, thereby generating a temporary skin factor. Unfortunately, field experience has shown that the wide range of performances required from diverting agents (selective, effective, and temporary plugging of the most-permeable/least-damaged zones) is very difficult to achieve.

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