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Interference TestInterference testing is one form of multiple-well testing. These tests are used to determine whether two or more wells are in pressure communication in the same reservoir and, when communication exists, to provide estimates of vertical formation permeability k and porosity/compressibility product ? Ct, in the vicinity of the tested wells.

In the homogeneous isotropic system, the porosity, thickness and the Permeability k is same everywhere and in all direction. Interference is conducted by producing from or injecting into one of these wells (active well) and the pressure response is observed in the other well (observation well). The active well starts producing at uniform pressure at time zero and the other pressure response in the observation well at a distance a from active well begins after some time lag.Introduction:To obtain maximum use of new reservoir analysis for under-standing local reservoir anomalies some improved methods are needed to describe heterogeneities in the reservoirs. Optimum well spacing Equipment specifications Well completion design Economics of pressure maintenance and secondary recovery programs etc depend on:

The extent and location of the areal heterogeneities that affect flow behavior in the field could be understood by interference test, constant-rate production or injection. During interference tests the flow characteristics of the formation are determined under in-situ. The method takes weeks or months to obtain a pressure response using conventional routine field operation during the test. 3IntroductionInterference Test is the kind of studies requires to understand the effect of pressure communication between the pair of wells in the reservoir. In this studies one well is called the observation well and other wells are called active wells. Obsn WellActive WellActive WellActive WellWell No 1 is observation well and Well 2, 3 & 4 are called active wells.In Well No1 we carry out the pressure buildup studies. The pressure response in well 1 is Influenced by changing the normal production of active wells.

4321 rwProcedure for conducting the Interference Test:For conducting the interference test in the reservoir in two or more than two wells one well called the observation well in which pressure variation is observed by changing the normal production of active wells or transmitter wells.

So for conducting the I/F test in a reservoir the pressure recorder is lowered in the observation well which was closed after achieving the constant rate q of production. Due to variation of flow rate of production in active wells, pressure impulses will be created in the observation wells. After the test is completed the pressure recorder is pulled out from the well. The change is pressure with time is determined from the interpretation of the pressure chart.Advantages of Interference TestTo determine the drainage area around the observation well. To optimize proper well space during field development stage for oil and gas exploitation Degree of inter communication between the wells. To delineate the communication and non-communication barriers faults/ pinch out between the observation and the active well in the reservoir. Degree of formation thickness Sharp variation in hydro conductivity / or preferential fluid pattern These studies will help the management to ascertain proper well space planning & undue investment for field development and thus to evaluate the implementation of effective reservoir management policy. The duration of Interference test is very long specially in tight reservoir and saturated oil reservoir .tt1Bottom Hole PressTimeTimeRate in Active welltt1Active wellObsen wellSchematic Illustration of rate history and pressure response for an Interference Test However, the radius of influence can be given by the following Equation;

rinf = 0.029 ( K t/ C) -----------------------------(1)

Theory of Interference Test:

As we know that the pressure variation in observation well is observed due to continuous variation in production of nearby active wells 2 & 3 is given by the following Eqn.

162.6 q B log (t+ t) q q1 - C a12 Pws = P* - ---------------- ----------------- + 70.6 ---------- * ---- Ei {---------------} + Kh t Kh q 0.00105 Kt1

q2 - C a22---- Ei { ------------------} --------------------------------------------(2) q 0.00105 Kt2

Where as q = production rate in the observation well before closure of the well.q1 = Production rate in well No. 2 and

q2 = production rate in other active well No. 3

The Eqn No. 2 clearly indicates that the log terms itself gives an effect of production and shut-in of the observation wells. The Ei terms indicates the pressure drops at the observation well caused by the production of active wells Well No2 & 3 located at a distance a1 & a2.

The above Eqn holds only:- When there is a single face fluid flowing in the reservoir below bubble point pressure. The mobility and the compressibility factor have to be used in for pressure buildup studies carried out in observation well during the interference test.Bottom Hole Pressure, PSIInterference Test in a low permeability Reservoir: Plot Pws Vs Log ( tClosed in time (t days)2.01.07.04.06.09.08.020.010.040.030.0Extra plotted Build-up pressure1400.01300.01600.01500.0 1800.01700.01200.01900.02000.0143 psiFig-1Closed in time (t+ t / t ) days100.010.01.01400.01300.01600.01500.0 1800.01700.01200.01900.02000.0Bottom Hole Pressure, PSIInterf. Test in a low perm. Res. Long time : Plot Pws Vs Log ( t+t/ tFig-2The Eqns No. 2 can be rearranged as under:

162.6 q B log (t+ t) q q1 - C a12 P* - ---------------- ----------------- - Pws = - 70.6 ---------- * ---- Ei ---------------- + Kh t Kh q 0.00105 Kt1

q2 Ei (- C a22)---- ------------------ --------------------------------------------(3) q 0.00105 Kt2

In this Eqn the first two terms in the LHS indicates the straight line portion of the curve and the third term Pws represents the observed pressure . So the above Eqn can be reduced to:

-m q1 - C a12 q2 - C a22 p = ----------- ---- Ei { -----------------} + ---- Ei { ---------------} + ------ 2.303 q 0.00105 Kt1 q 0.00105 Kt2

That is : Press Extra plotted- Press observed = Cal pressure dropRemarks:Mini period of two months is required for conducting the test where the reservoir is having very low permeability or tight reservoir. The actual production condition of the active well need to be varied to create the pressure impulses in observation well during the Interference Test.

Qns:

If q= 140 bbls/day in observation well m= 270 psi/ cycleC= 6.9 x 10 -6 / psiq1= 180 bbls/d a1= 1835 ftC= 10-6 ( Assumed)

From the above data find the pressure in a observation well.

Ans:

-m q1 { - C a12 } q2 { - C a22 } p = ----------- ---- Ei ----------------- + ---- Ei ------------------- + ------ 2.303 q 0.00105 Kt1 q 0.00105 Kt2

= -270 180 (10-6 * (1835)2 ------- -------- Ei --------------------- 2.3030 140 0.00105 * 3070

p = 32 psi Ans.

from this data a graph is plotted between the Pws Vs Log (t+ t)/ t. The dash line indicates the extrapolated build-up pressure is obtain by extrapolation of the Log plot. The difference between the extra polated dash line and the actual line of pressure buildup (Pws Vs Log t ) will indicate the actual pressure drop in an observation well (Fig-1).Problem: 2Find the value of fluid transmissibility and reservoir storativity & porosity between the wells considered for interference test with= 140 bbls/d the following reservoir data:q = 140bbls/dbeta= 1.1m= 270cycle/psic= 6.9x10-4/ psiq1 = 180bbls/d c = 3.5x10 -7 Soln: since fluid transmissibility (kh/)= 162.6qB/m Ans: 92.6 c kh 1The value of h = ------------ ----- x ------ Ans :4.70 & Porosity= 0.11 k c

Analysis of Interference Test Based on Type curvesIntroductionThe reservoir input data could be analyzed for better reservoir characterization. These are the standard curves are generated by simulating constant rate pressure drawdown, fall-off, (or injection) and also applied for build-up tests if an equivalent shut-in time is used as a time variable on the graph. The type curves are advantageous because they may allow the test interpretations even when the well bore storage distorted most or all the test data in that case , conventional methods failed. Fundamentally the types curves are pre-plotted family of pressure draw down curves. Most fundamental of these curves are of Rameys. This further signified that it is plot of dimensionless plot of pressure data PD vs Dimensional change in time tD. These types of curves generated were also found useful to interpret the interference test to generate various petro physical parameters for better reservoir characterizationtype curve superimpose.pdfThe type curves generated from the simulation approach were also used for the interpretation of the Interference test also.Log-Log PlotPDtD/ rD210-110-110-210-310-410-310-210-4Procedures:Plot the pressure draw down data p = (Pi-P) vs Elapse time (t) on the same size of Log-Log plot as full scale as shown above. Slide the plot test data over the type curve until the match is found ( Horizontal & vertical sliding,. Both required).Record pressure and time match points ( PD)MP, (p)MP, (tD/rD2)MP and tMP

where (MP)= Match Point

141.2 q ( PD)MPCalculate the permeability (K)= ----------------- --------------- h ( P)MP

0.0002637 K tMPCalculate c= ------------------ ------------ rw2 (tD/ rd2) MP

0.00708 Kh (Pi-P)The value of PD= ------------------------------- q

Problem:The I/F test was run in a shallow water sand reservoir the active well is producing 466 BWPD and pressure response in observation located 99ft from the active well was measured as function of the time elapse since the drawdown in the active well. Find the value of Ct with the following data:

w= 1cp Bw= 1.0 rb/stbH= 9 ftRw= 3 Porosity= 0.3 The pressure data are in the following table.Delt ( Min)Pws (psi)Delp= PI-pwf0148.9205148.92025144.914.0140143.725.250143.185.74100141.477.45200139.729.2300138.710.22400137.9910580137.1211.8Solution:p= Pi-Pwft (min)Interference Test data from water ReservoirSolution:Assuming that an aquifer is homogeneous, isotropic and infinite acting. Here we use Ei- function type curves. The pair of match point are:

t= 128 min(tD/ rd2) MP= 10p= 5.1psi PD= 1.0

141.2* 466& 1.0*1.0 1.0Then the value of K= ----------------------------------- ---------- = 1433 md 9 5.1

Solution:Assuming that an aquifer is homogeneous, isotropic and infinite acting. Here we use Ei- function type curves. The pair of match point are:

t= 128 min(tD/ rd2) MP= 10p= 5.1psi PD= 1.0

141.2* 466& 1.0*1.0 1.0Then the value of K= ----------------------------------- ---------- = 1433 md 9 5.1

0.000264* 1433* 128/60 Ct= ---------------------------------------------- = 2.74x 10-5/psi 0.3* (99)2 * 1.0*1.0

some of the types curves are shown in the following figures:Figc1.pdffigc2.pdffigc3.pdffigc4.pdfThanks