COMPLIANCE INFO - Laserfiche WebLink

Home Browse Search

SPE 16798 PRAMOD K. SINGH, RAM G. AGARWAL AND LOREN D. KRASE 7 straight line drawn through steps after wellbore 4. Data influenced by wellbore storage and fillup has an intercept much above the pretest p.-es- changing wellbore storage will plot with a con- sure point (pref 1060 psi at zero rate) suggesting cave upward curvature on the p vs. q plot. It that these steps are above the parting pressure. is incorrect to force a straight line through This is confirmed by the Odeh and Jones multirate these steps. analysis plot shown in Fig. 24. FPP is therefore interpreted to be 2169 psi, the pressure at the end 5. Multirate analysis can be applied to determine of the third step. the parting pressure provided the time step size is at least long enough to be in a 2. Field Example B storage-radial transition flow regime. Accu- rate kh analysis, however, requires the data to This illustrates the use of linear flow multi- be in a purely radial flow regime. rate analysis to determine parting pressure for a fractured well. The well was shut-in for a falloff 6. The rate-time history before a SRT and the test just before the SRT. Analysis of the falloff pressure at the start of the SRT should be data indicated a uniform flux fracture of 110 ft recorded for valid multirate analysis. The half-length. The SRT consisted of 1-hour time steps proper application of multirate analysis has of approximately 500 bpd rate increments. been demonstrated. The p (surface pressures, corrected for frit- 7. Linear flow superposition is a powerful tool tional pressure loss) vs. q plot is shown in for determining parting pressure from SRT data Fig. 25. Data for the early steps form a concave on a fractured well. Shorter time steps, lim- upward curvature. This behavior may be observed ited to the duration of Linear flow, can be simply due to superposition effects even for steps used in this case. with linear flow. Based on the interpretation (two straight lines) shown in Fig. 25, parting pressure B. The techniques developed in this paper have would be interpreted to be between steps 13 and 14, been successfully applied to field SRT data. at 975 psi. NOMENCLATURE The radial flow Odeh and Jones plot for this case did not give a clear indication of parting b' = intercept of line from Odeh and Jones anal- pressure and is not shown. Fig. 26 is the linear ysis (see Eq. 3) flow Odeh and Jones plot. The falloff before the SRT is accounted for as a transient step and pres- B = formation volume factor, RB/STB (res m3/stock sure at the end of the falloff (p FF) is used for tank m3) superposition. Clearly, data for rtge first ten steps fall on a single curve. A shift in data for c = total system compressibility, psi 1 (kPa 1) steps beyond step 10 indicates that these steps are above the parting pressure. The true FPP is there- C = wellbore storage factor, RB/psi (res m3/kPa) fore interpreted to be 744 psi, the pressure at the end of the tenth step. FCD = dimensionless fracture conductivity Note that for this example the p vs. q plot h = formation thickness, ft (m) (Fig. 25) does not show a definite change in slope corresponding to the FPP. In fact, data points for k = formation permeability, and all the steps form a smooth S-shaped curve. Interpretation for parting pressure from this plot m' = slope of line from Odeh and Jones analysis is, therefore, very subjective. Clearly, linear (see Eq. 2) flow multirate analysis is a superior analysis tool for this case. p = pressure, psi (kPa) CONCLUDING REMARKS Pref = pressure at the beginning of a rate change (beginning of the SRT, see Fig. 14), psi A systematic investigation of a number of sig- (kPa) nificant factors affecting SRT design and analysis has been made. Based on the results of this inves- Ap = pressure change, psi (kPa) tigation, the following conclusions can be made: q = flow rate, negativefor injection, STB/D or 1. Previously available guidelines for SRT proce- Mscf/D ("standard" m3/D) dure and analysis are not adequate. q = long term stabilized rate prior to the SRT 2. Equal size time steps of length long enough to L (see Fig. 14), STB/D or Mscf/D ("standard" be out of wellbore storage should be used for m3/D) an ideal SRT. q = reduced or changed rate prior to the SRT (see 3. Data completely dominated by wellbore storage r Fig. 14), STB/D or Mscf/D ("standard" m3/D) will plot with a unit slope on a log-log plot of dp vs. cumulative injection (production) for r = radius, ft (m) a constant wellbore storage factor. s = skin factor 497