High Grading Fracture Candidate Selection
Authors: Krasey R. 
Published: March 2010

Abstract
The ability to choose the best possible fracture stimulation candidate is very important in order to maximize production and revenue, To compare wells consistently in a given area, pressure build-up techniques are utilized. An up-lo-dale isobaric map is required. An estimate of the slope m of all the wells in the area is then determined using known m's from build-up and Jail-off tests. Since Pr and m are given/or all the wells, build-up calculations can be done to determine skin factors and maximum anticipated low rates with a −4 skin factor. Once maximum rates for oil have been estimated the wells with the greatest variance between actual oil production and estimated −4 skin oil production are investigated further to determine the best fracture candidates.

Introduction
The following paper outlines a method for high-grading fracture candidate selection in mature waterfloods, with medium to low permeability and pressures above the bubble point, by the use of pressure build-up surveys and injection well fall-off tests. The area investigated is the Pembina Easyford Cardium Unit No.1 located 15 km northwest of Drayton Valley. The unit is operated by Canadian Occidental and consists of 15 water injection wells, 78 producing wells and 15 shut-in wells.
To fulfill Energy Resources Conservation Board's (ERCB) requirements, an extensive pressure build-up program was conducted from September to December 1986. In all, 17 wells were placed on build-up. To complement the pressure build-upsurvey all 15 water injection wells had fall-off tests conducted on them. In addition, three shut-in wells had injection fall-off tests performed on them (Fig. 1).
The high-grading method calculates the present skin factor and then calculates the stabilized production rate at a skin = −4 for all the producing and shut-in wells within the unit. The method depends on assigned values of Pr, derived from the build-up surveys, fall-off tests and static gradients, and the Horner slope m for each well. The criteria and limitations will be discussed in a following section. The final step in the high-grading method compares the actual present oil production rate for each well with the estimated stabilized production rate at a S = −4. The wells with the greatest increase in oil production rates are the wells which should be investigated as potential fracture stimulation candidates.
It should be emphasized that the method is only a screening method for determining the best possible fracture stimulation candidates. Once a short list has been generated, other criteria such as WOR, decline rate, peak fluid production rate, remaining recoverable reserves, well recovery and fracture stimulation history can be used to determine the best candidates for fracture stimulation.

Equations
The following equations were used in the skin and stabilized rate calculations on all of the producing and shut-in wells in Easyford.
Equation (Available In Full Paper)
The actual reservoir conditions behave in between both cases.
The constant was calculated by using the stabilized rate equation and setting q5=q0 and S equal to the calculated present skin factor equation.