Conditions Favourable to Infill Drilling in Waterfloods in Sandstone Reservoirs Containing Medium/Heavy Oils
Authors: Singhal A.K. , Springer S.J. 
Published: March 2010

Abstract
Production histories of four waterfloods, each with several infill wells, were examined to identify conditions favourable to infill drilling in sandstone reservoirs containing heavy/medium oils under waterflood. The dominant role of reservoir heterogeneity was inferred from the nature of performance of newer and older infill wells. Due to these heterogeneities, some of the mobile oil contained within the reservoir is not drained by the older wells, but infill wells could drain a portion (up to 25%) of this oil.
Based on the cases studied, conditions favourable for infill wells were:Current oil rates of older wells greater than 3 m(3)/d and watercut less than 75%.Original Oil-in-Place (OOIP)/well for the reduced spacing greater than 80 E3m(3) and total recovery factor (primary + waterflood) greater than 25%.
It is recommended that these criteria be validated by further technical and economic studies.

Introduction
Infill drilling was initially proposed as a means of draining incremental reserves, besides accelerating established oil drainage, based mainly on geological considerations and simulation studies(1–4) mostly dealing with the carbonate reservoirs of Texas. This work aims to obtain clues for identifying attractive infill opportunities in clastic reservoirs containing medium/heavy oils under waterflooding. Our expectation was that, with the growing database on performance of infill wells of recent years, some of the initial premises could be revisited and a fresh attempt could be made to obtain additional insights.
Intuitively, as infill wells are brought on stream, they would accelerate oil production, depending on the amount of mobile oil remaining within the drainage region. They may also be able to drain oil from certain poorly connected parts of the reservoir that would otherwise be left unrecovered. They should, therefore, add value to the ongoing waterflood. It goes without saying that value added (returns) should be consistent with the risks (investment) involved. This study focuses on production performance of infill wells in four different waterfloods in heavy/medium oil sandstone reservoirs of Alberta, Canada.
It has been suggested(3) that infill wells enable better pattern control and connectivity in waterfloods, shorten life and improve areal and vertical sweep by the injected water. Therefore, infill well's performance should largely depend upon reservoir heterogeneity, besides factors such as mobility ratios, average ultimate recovery factors and maturity of exploitation reflected in the prevailing oil rates and watercuts. Reservoir heterogeneity in the vertical direction is usually characterized by Dykstra-Parson's coefficient. For lateral or aerial heterogeneity, a method based on performance of previous infill wells was recently proposed(5, 6). This method involves plotting cumulative oil production versus frequency data for previous infill wells from the same (or analogous) project on a lognormal graph, and examining the slope of the curve around the median (50% probability). The basic assumption here is that reserves from contemporaneous infill wells, like the distribution of many geological parameters, should follow a lognormal trend. However, significant deviations from the lognormal distribution are usually observed (e.g. Figure 1).