Performance of the Skewed Four-Spot Injection Pattern

Abstract

Abstract Secondary recovery projects often are not started in oil reservoirs until dictated by rising GOR's or declining oil production. Such circumstances require a well dispersed production. Such circumstances require a well dispersed injection pattern to prevent the oil bank from bypassing the production wells. Many times this purpose is served by using the five-spot or line-drive patterns, in which half of the wells are made injection wells. A regular injection pattern is proposed that is based on the same well spacing pattern is proposed that is based on the same well spacing as the five-spot but in which only a third of the wells need be converted to injection wells. Fluid flow model studies on this proposed pattern show that production histories will be as good as those for a five-spot pattern at mobility ratios of three and above. The model data are presented in a form that may be used to predict recoveries from both homogeneous and stratified formations. Introduction The injection of fluid into a petroleum reservoir usually and pumped by an actual, or predicted, loss in the productivity of the reservoir. Often, an oil reservoir is productivity of the reservoir. Often, an oil reservoir is produced by pressure depletion until high GOR's dictate a produced by pressure depletion until high GOR's dictate a change in policy. Fluid injection also causes increased ultimate recovery, but this usually is of less importance to the operator than is the maintenance of the production rate. It is a fact that the ultimate recovery in any particular fluid injection project is relatively independent of the number and spacing of the injection wells as long as the reservoir voidage through the production wells is balanced by the total injection rate. However, the response of a production well is directly affected by the distance it lies from production well is directly affected by the distance it lies from an injection well. In places where the well productivities are declining, the injection wells should be spaced as evenly as possible among the producing wells ("dispersed" injection pattern). More particularly, if the production response depends upon the formation (and subsequent production) of an oil bank, each production well should be surrounded by injection wells. Otherwise, the oil productivity will increase very little as the oil bank moves productivity will increase very little as the oil bank moves past the well. An evenly spaced injection pattern has the past the well. An evenly spaced injection pattern has the best chance of generating an oil kick in the production wells. If reservoir voidage and injection rates are to be equal, the ratio of injection wells to production wells must be equal to the ratio of the average injectivity to the average productivity (in reservoir volumes). This ratio is productivity (in reservoir volumes). This ratio is instrumental in choosing the injection pattern. Table 1 shows the production well to injection well ratio for the dispersed injection patterns that have been described in the literature. All of these patterns will have comparable recoveries and response times (for the same basic distance between wells). The choice of pattern therefore depends upon the relative injectivity and productivity of the wells and upon the type of drilling pattern employed. Another important factor may be the ability of the proposed production wells to make up the lost productivity proposed production wells to make up the lost productivity from those wells converted to injectors. These factors become more pronounced at the higher oil viscosities (low gravity oils). All other things being equal, the probable ratio of single well injectivity to single well productivity will be equal to the water-oil mobility ratio of the proposed flood. Thus, high mobility ratios (due to low gravity proposed flood. Thus, high mobility ratios (due to low gravity oil) should call for fewer injection wells - as long as these injection wells are "dispersed" to obtain more even production response for the remaining wells. Secondary production response for the remaining wells. Secondary recovery projects in oil reservoirs seldom, if ever, will have mobility ratios requiring more injection wells than production wells. Thus, only the last five of the patterns listed production wells. Thus, only the last five of the patterns listed in Table 1 will be of significant interest in oil recovery. TABLE 1-CHARACTERISTICS OFDISPERSED INJECTION PATTERNS No. of Production Drilling PatternPattern No. of Injection Required Nine-spot 1/3 Square Seven-spot 1/2 Equilateral triangle Straight line drive 1 Square Staggered line drive 1 Offset lines of wells Five-spot 1 Square Four-spot (inverted 2 Equilateral triangleseven-spot Inverted nine-spot 3 Square JPT P. 1315