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SPE 16798 PRA.MOD K. SINGH, RAM G. AGARWAL AND LOREN D. KRASE 5 require equal rate increments for a preparting reduced constant injection rate. Moreover, the straight line. A radial flow SRT with no wellbore shut-in period is seldom Long enough to obtain a storage or skin was simulated to see the effect of uniform static pressure, lp. , throughout the drainage changing rate increments. Although not shown, a area. The question arises as to what pressure value straight line was obtained on the p vs. q plot. to use for multirate analysis. Felsenthal2 suggests using the intercept of the preparting straight line Rate increments should be planned to provide at on the p vs q plot when q=0 forp. . We find this least 4-5 steps before an expected parting pressure criterion can give a false indication of parting is reached to establish a good preparting straight pressure especially if data are influenced by line on the p vs. q plot. For example, Bennett storage effects. et al.3 used a radial reservoir model with best available reservoir properties and initial condi- To determine the proper value of the pre-SRT tions for their SRT design. pressure and to establish what pre-SRT time periods are important for the multirate analysis, two cases Rate controllers should be used, if possible, were simulated: (1) SRT fallowing a shut-in period, to maintain constant rates during each step. Actual and (2) SRT following a stabilized injection period. injection rates should be recorded throughout the These cases are somewhat analogous to the field con- test along with continuous bottomhole pressure meas- ditions and are schematically shown in Fig. 14. The urements to enable a good analysis of the SRT data. pressure at the beginning of the SRT is labelled 5. Skin Damage Pref' 1. SRT Following a Shut-In Period Skin damage causes an additional pressure drop in the near the wellbore region and reduces the This consists of a constant long-term injection pressure which is actually applied to the formation. rate, q, (Fig. 14) for 14 days starting with an ini- Although not shown here, if the skin damage is tial static reservoir pressure of 1000 psi. The removed due to high injection rates during a SRT, a well was then shut in and followed by a four-step shift in data will be observed on the p vs. q plot. SRT of 1 hour time steps. To study the impact of Data for the remaining steps, however, will fall on the duration of the shut-in times, two cases with another straight line with a reduced slope. How- shut-in periods of 3 and 48 hours were simulated. ever, this line should extrapolate back towards the pretest pressure-rate point for a no fracture exten- Odeh and Jones multirate analysis was performed sion case. Also, a downward shift in data will be (for 48 hour shut-in) using p ff for p. and ignoring observed on the Odeh and Jones multirate analysis the rate-time history prior toeEhe SRT; This plot corresponding to the step where the skin damage involves performing superposition starting with time is removed, but the data for the remaining steps at the beginning of the SRT (to, Fig. 14). Although will not show a continued downward shift. It is not shown, data for all the steeps fell on a single therefore possible to distinguish between the semilog straight line with the correct slope. This removal of skin damage and exceeding the FPP if they suggests that if the shut-in period is much longer occur as separate incidents during a SRT. than the SRT step length' Pref can be used as p. for multirate analysis, and the pre-SRT rate-time his- ADDITIONAL CONSIDERATIONS FOR MULTIRATE ANALYSIS tory can be ignored. A single straight Line is, however, not obtained for a similar analysis for the The use of muLtirate analysis requires a value case of a 3 hour shut-in as can be seen in Fig. 15. of pressure and rate at the beginning of the test. It will be shown that if a proper pressure value is The data for the SRT following a 3 hour shut-in not used, misleading conclusions may be reached was reanalyzed assuming a pseudo-steady state or regarding the FPP. The Odeh and Jones superposition steady-state (pss/ss) condition to have been method$ described earlier inherently assumes that achieved during the initial 14 day Long-term injec- the entire reservoir (drainage area) is shutin and tion period. Here, the 3 hour shut-in period is stabilized at p. before the start of the test. A included as a transient step, i.e., the superposi- single preparting straight line is obtained if tion time function is calculated starting with time, superposition is applied with the correct value of t a, at the beginning of the shut-in period. How- p. (=1000 psi) for the simulated SRT case shown in ever, p is used for p. in calculating the pres- Fig. 3. sure function ion for the SR� data. The data now falls on a single straight line with the correct semilog The effects of using lower and higher pressures slope as shown in Fig. 16. than the actual are shown in Figs. 12 and 13, respectively. Fig. 12 is an Odeh and Jones plot of 2. SRT Following a Stabilized Injection Period the same data using a p. of 900 psi. A downward shift in data for all tAe steps is observed, which A three-step prefrac SRT was simulated can be misinterpreted as if all the steps are above replacing the shut-in period by a 7 day reduced parting pressure. Fig. 13 is a multirate plot of injection rate (q ) period (Fig. 14). Here, the the same data using a p. of 1100 psi. Here, an pressure at the end of the 7 day injection is p upward shift in data islobserved for each successive ref' preparting step. Data for steps above parting pres- Results of the Odeh and Jones multirate anal- sure, however, indicate the downward shift of data. ysis using p f for p. and ignoring the pre-SRT rate history is soown in Fig. 17. Note that data for In actual practice, SRTs are usually run after each step plots with a different slope and none of either shutting in the well or stabilizing it at a them represent the correct semilog slope. This is 495