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SYSTEMATIC DESIGN AND ANALYSIS OF STEP RATE TESTS TO 2 DETERMINE FORMATION PARTING PRESSURE SPE 16798 proper application of multirate analysis methods; (4) a method for determining parting (propagation) pressure from SRT data on a vertically fractured B r well; (5) application of the developed analysis Pw = pe- [141.2 kh (Ln r.e + s)] q .........(1) techniques to field data; and (6) guidelines to w design and conduct successful tests in the field. To aid this study, a single-phase, two- dimensional, finite-difference gas reservoir model If a steady-state condition is achieved during was used in a Liquid mode to simulate SRT on a each injection step, r and pe will be constant. A single well in an infinite reservoir. Fracture Linear relationship wigl therefore exist between half-lengths were increased arbitrarily in discrete pressure (p) at the end of each constant-rate injec- Lengths to approximately simulate fracture extension tion step and the corresponding injection rate (q). above the FPP. This limitation, however, does not alter the conclusions of this study. The analysis For most SRTs, ste? time lengths are seldom methods presented in this paper are not meant to long enough to reach a steady-state condition. In characterize the fracture propagation mechanisms but this case r can be replaced by rd, the drainage to demonstrate the influence of various parameters radius as defined by Aronofsky and Jenkins. If on preparting SRT data and on identification of the time step size is constant, a linear relationship FPP. should still exist between p and q for the test data below parting pressure. This is because the logar- REVIEW OF TESTING AND ANALYSIS METHODS ithmic term Lnr /r will be insensitive to small changes in rd duriingwthe test. Testing Procedure A cartesian plot of p vs. q is made for the SRT The SRT procedure is schematically shown in data. When the formation face injection pressure Fig. 1. Generally, the test well is either shut-in exceeds parting pressure, the resulting fracture or stabilized at a reduced but constant injection acts as an additional fluid conductor. This changes rate prior to the start of the SRT. Ideally, the (reduces) the slope of the p vs. q curve accord- shut-in period should be long enough so that the ingly. This is illustrated by the p vs. q plot of bottomhoLe pressure is near the static formation the simulated data as shown in Fig. 2. Since a gas pressure. Alternatively, if the well is stabilized reservoir model, in an equivalent liquid mode, was at a reduced injection rate, the stabilization utilized for this study, gas units (Mcf) and liquid period should be Long enough to achieve a steady- units (Bbls) have been interchangeably used in state (ss) or a pseudosteady-state (pss) condition. Fig. 2 and elsewhere in this paper. The preparting The SRT that follows, consists of a series of data, including the pretest pressure of 1000 psi, constant-rate injections with rates increasing from falls on a straight Line. Normally, another low to high in a stepwise fashion. Each constant- straight line is drawn through the points above the rate step is normally of equal time length. Injec- parting pressure as shown in Fig. 2. The pressure tion rates and pressures are recorded for each step corresponding to the point where the two Lines and analyzed to determine the FPP. intersect is interpreted as the parting pressure. This method provides an approximate estimate of Analysis Methods parting pressure. Since the fracture length con- tinues to increase above the FPP, there is no theor- To validate the model and demonstrate the etical basis for drawing a second straight line assumptions and limitations of the existing analysis through the points above the parting pressure. How- techniques, a six-step SRT with equal length of time ever, on the vertical axis, this line extrapolates steps (also referred to as time step size) and rate back to a pressure point which is much higher than increments and no wellbore storage or skin was simu- the pretest pressure. The value of the intercept Lated. The first three steps represent transient with respect to the pretest pressure provides a qua- radial flow below parting pressure. To simulate Litative indication that the pressure points corre- fracture extension, an infinite conductivity frac- sponding to the second straight line are above the ture of arbitrary half-length, 2.5 ft, was intro- parting pressure. duced at the beginning of the fourth step and subsequently increased to 5.0 ft and 7.5 ft at the (ii) Multirate Analysis beginning of the fifth and sixth steps, respec- tively. Note that these fracture lengths were If pressures are recorded with a continuous chosen arbitrarily simply to demonstrate the anal- readout device and accurate injection rate data is ysis methods and establish fracture propagation obtained, multirate pressure transient analysis during steps above the FPP. Actual fracture lengths techniques can also be applied to SRTs. This tech- attained during the field operations are anticipated nique is based on the principle of "superposition." to be substantially Longer. This, however, should Application of the Odeh and Jones superposition not affect the interpretation methods for the FPP as method$ to SRT data has been presented in the liter- outlined in this study. ature.2" The Odeh and Jones method assumes tran- sient radial flow into the reservoir during each (i) p vs. q Plot constant rate period. A plot of The conventional analysis method assumes a steady-state Darcy flow into an injection well,e and is based on Eq. 1. 492