New Approach For Analysis Of Pressure Fall-Off Tests In Fractured Carbonate Waterflood Reservoirs
Authors: El-Hadidi S.M. 
Published: December 2007

A method of calculating wellbore damage an fracture lengths from pressure fall-off data is presented. The method is based on a linear flow presented. The method is based on a linear flow model that simulates conditions present during the early-time period after shutting in an injection well The proper choice of the straight line representing linear flow is quite significant. A procedure is outlined to secure the proper choice of the straight line.

Pressure fall-off test data from three injection wells representing three basic cases in fractured carbonated reservoirs were discussed and results obtained from applying the method are presented. presented. Surface pressure fall-off test data were recorded using telemetry in one of the injection wells. Dead weight tester was also used to check the accuracy of telemetry readings. Bottom hole pressure data were recorded using bottom hole pressure data were recorded using bottom hole pressure gauges in the same well to establish pressure gauges in the same well to establish the validity of using surface pressure measurements for pressure fall-off analysis. Both surface pressure measurements and bottom hole pressure measurements and bottom hole measurements were analysed.

The presence of skin around the wellbore or the fracture face in natural or induced fractured reservoirs influence the slope of the straight line resulted from plotting pressure difference versus shut-in time on log-log plot. If there is no skin, a straight line of slope half is obtained. On the other hand, straight line of slope less or greater than half should be obtained in cases of damaged or improved conditions, respectively. Correcting the pressure fall-off data for the pressure change due to skin and plotting the corrected pressure difference versus plotting the corrected pressure difference versus shut-in time on log-log plot will secure the straight line of slope half. This criterion is the one used to secure the proper choice of straight line representing linear flow.

Introduction

The presence of natural or induced fractures will influence the flow behaviour in reservoirs. In a fractured system, flow in the region nearest the fracture is practically linear, whereas, farther away from the fracture, essentially radial flow previals. Thus, transient pressure analysis based on radial flow theory are incorrect when applied to early time data where linear flow predominates. predominates. A linear flow will show as a straight line on a pressure difference versus square root of shut-in time. The slope of the line is inversely proportional to the surface area of the fracture. proportional to the surface area of the fracture. Linear flow will also show a straight line of half slope on a plot of the logarithm of the pressure difference versus the logarithm of shut-in time. The presence of damage around the wellbore or on fracture surface could prevent or distort the line of half slope on the log-log plot. The only way to secure a straight line of half slope is to correct pressure differences for the pressure drop due to skin damage or for pressure increase due to improved condition.

Interpretation guidelines are presented for four cases in fractured carbonate water injection wells. The first case is one where delta Pskin = 0 (no skin), the second case is one where delta Pskin greater than 0 (skin damage), and the third case is one where delta Pskin less than 0 (improved condition). In the fourth case, bottom hole pressure data and surface pressure measurements are presented to establish validity of using surface pressure measurements for pressure fall-off analysis.

THEORY AND WORKING EQUATIONS

In general, all pressure transient tests are based on some form of the diffusivity equation which expresses relationship between pressure, time and distance for a particular system.