Produced Water Reinjection Performance Joint Industry Project TerraTek, Inc. Triangle Engineering Taurus Reservoir Solutions (DE&S) E-first Technologies AdvantekVIPS

Calculation of injectivity index (II) Motivation: II is used internally by companies to report and compare data, our aim is to correlate II with PW characteristics It is necessary to use consistent definitions Principle: II is supposed to be a parameter characterizing the injectivity which is “constant” for a given completion and can be used to predict rate or pressure

Standard definition of II (matrix) Q = inj. Rate P bhi, P e = BH inj. And outside radius pressure k w h i = water perm x inj. Height r w, r e = wellbore and outside radius S = skin (account for completion, fracturing)

Standard definition of II Assumptions: Matrix (radial) flow Single phase, pressure maintained at outside radius No thermal effects Etc. etc.

Proper use of parameters (best practices for matrix II evaluation): n Use effective TOTAL mobility of fluids: k ( k rw /  w + k ro /  o ) k ( k rw /  w + k ro /  o ) n Use effective injection kh, if well test available, adjust accordingly n Use some “average” values for viscosities to account for temperature effects (weighted towards cooled region) n Do not include induced fracture in the skin (except propped/acid frac)

What do we use the above II for: n Formula gives EXPECTED II without plugging and S effects – maximum possible injectivity n If we exclude S, allows us to determine total skin from comparison of measured and calculated (theoretical) values n By including various parts of S hopefully we will be able to separate plugging and mechanical (geometry) skin components

II in fracture injection regime n No longer linear relationship between Q and (p wfi – p e ) n Pressure controlled by fracture mechanics: n P bhi = S min +  P compl +  P net = P f –S min = minimum stress = P foc –  P compl = pressure drop through completion –  P net = net pressure in the fracture n May be again linear BUT with (P f – P foc )

II in fracture vs matrix injection regime

Definition of II in fracture inj mode –Matrix definition II = Q / (P bhi – P e ) gives VARIABLE II depending on rate –Correct definition is “differential” : II f = [(Q 2 – Q 1 ) /(P 2 – P 1 )] f –Another expression (for predictions): P wf = P f0 + II f (Q – Q f0 )

II in fracture mode

Calculation of II from P,Q vs time data (observed II) –Matrix mode: customary calculation is correct II n = Q n /(P wf n – P e ) –Fracture mode: customary calculation is incorrect and will UNDERESTIMATE actual II –Best way to calculate not obvious –Conventional evaluation may appear to give a “constant” value because of operation constraints

Example: Heidrun B3H

Example: Heidrun B3H conventional II

Methods for calc of II in fracture regime n Take subsequent time series data: II n = (Q n – Q n-1 )/(P n – P n-1 ) - this produces large scatter n Use Hall plot data: II n = (QSUM n – QSUM n-1 )/(PSUM n -PSUM n-1 ) - is equivalent to the MATRIX (wrong) formula n Use cumulative Hall data: II n = QSUM n /PSUM n - averages the previous (also incorrect)

II methods for fractured inj

Recommended method for calc of II in fracture regime n Evaluate fracture p vs Q trend from p vs Q plot - INTERPRETATION n Determine intercept P int at Q=0 n Calculate II by II n = Q n /(P n – P int ) n Discard values if (P n – P int ) < (P f0 - P int ) – this will eliminate matrix data.

a) Intercept and slope from p vs Q

b) Calculate II

Conclusions n Conventional calculations should not be used in fracture mode (underestimates II, rate dependent) n Proper determination requires independent analysis of slope and intercept in P vs Q plot n Serious problems can result if conventional II is used to develop trends and correlations (e.g., II as a function of water quality and k …)