Two-Step Rate Test: New Procedure for Determining Formation Parting Pressure

Abstract

Summary. A reliable estimate of the formation parting or fracture extension pressure is important for efficient operation of waterfloods and tertiary recovery projects. A new procedure, called a two-step rate test procedure, called a two-step rate test (2-SRT), is presented for determining this pressure. Unlike a conventional step rate test (SRT), which consists of several constant-rate injection steps, the 2-SRT requires only two injection steps, during which pressures are measured continuously. Test procedure, design and analysis procedure, design and analysis considerations, and recommendations for test implementation are provided. Field examples are included. The 2-SRT procedure offers potential for time procedure offers potential for time and cost savings over a conventional SRT. Introduction Rapid water breakthrough in production wells as a direct result of exceeding a certain critical injection pressure in nearby injection wells was observed as early as 1945. This critical injection pressure is called the formation or fracture parting pressure (FPP). The FPP is equivalent to the "fracture extension/propagation pressure" in the hydraulic fracturing literature. Several studies recently demonstrated that a fracture will continue to propagate if injection is above the FPP and the injection/ withdrawal ratio is greater than one. In addition to this uncontrolled fracture extension, injection above the FPP may also cause fracturing out of pay. These factors may lead to premature breakthrough of injected fluids, poor sweep efficiency, reduced recovery, poor sweep efficiency, reduced recovery, and loss of costly injection fluids. On the other hand, injection far below the FPP may result in injection volumes much lower than the allowable maximum and a reduced rate of oil recovery. A reliable estimate of the FPP is therefore critical in conducting secondary and tertiary recovery projects. The SRT has been the primary method used for several years to determine FPP. A recent paper provided a detailed discussion of the design and analysis of SRT'S. To define the FPP adequately from an SRT, several (usually seven or more) constant-rate injection steps, generally of equal duration, are needed. Further, the duration of each step (also referred to as timestep size) should ideally be long enough for the data to be free of wellbore-storage effects. If continuous pressure measurements are available during pressure measurements are available during an SRT, the data can be analyzed by multiple-rate superposition methods to determine the FPP. The Odeh and Jones superposition method or Agarwal's multirate equivalent time analysis can be used to analyze SRT data. Ref. 7 gives a detailed discussion of the proper application of multiple-rate analysis and identification of FPP from such an analysis. This paper describes a new test procedure (2-SRT) for determining FPP. The 2-SRT requires only two constant-rate steps, during which pressure and time data are measured continuously and analyzed for determination of the FPP by multiple-rate superposition methods. Pressure data from the two steps are compared on Agarwal's multiple-rate equivalent-time basis. Guidelines for the design and analysis of a 2-SRT and recommendations regarding the test implementation are provided. Field examples are included to demonstrate the applicability of this procedure. procedure. Discussion Test Procedure. The 2-SRT can be run in at least four different modes shown schematically in Fig. 1. For each mode, the test well is either shut in or stabilized at a constant injection rate before the start of the test. In either case, the stabilized pressure before the 2-SRT must be below the FPP. Modes 1 through 3 refer to cases where the well is stabilized at a constant injection rate before the test is run. Mode 4 is the case when the well is shut in at the time of the test. The 2-SRT procedure for each mode is described below. Mode 1. The injection well is shut in for Step 1, followed by injection at a high constant rate for Step 2. The injection rate for Step 2 is chosen such that the FPP will be exceeded. Mode 2. If the well is on a stabilized low-rate injection, Step 1 of the test may be stepping up the injection rate to a constant value that is still low enough so that the FPP is not exceeded. The injection rate is then further increased to a higher constant rate for Step 2. Mode 3. The injection is reduced to a lower constant rate for Step 1 and then increased to a higher constant rate for Step 2. Mode 4. The well is stabilized at shut-in conditions. The 2-SRT consists of two constant-rate injection steps with a stepwise increase in the injection rate for each step. For all the modes of the 2-SRT procedure, it is extremely important that the pressures attained for the entire duration of Step 1 are below the FPP. The injection rate for Step 2 is chosen such that the injection pressure will exceed the FPP during this step. JPT P. 84