A Petrophysical Method to Estimate Production Rate vs.Time Performance,and to Define Drainage Areas and Recovery Efficiencies

摘要

Objective/Scope: The main objective is to build on prior publications of decline curve analysis,based ontheoretical producing and fluid flow characteristics(Matthews and Lefkowits 1956,Fetkovitch 1975,Cox1948).In this paper,we relate petrophysical predictions of ultimate oil recovery to actual rate-time wellperformance by adjusting recovery efficiencies and well-bore drainage areas.Method/Procedure/Scope: From an analysis of a number of producing wells in the Permian Basin a model(Zimbrick)has been derived for average decline curve performance.This model has been found to work inother basins; however,basin specific models are likely to be required in some situations.This model is then combined with petrophysical estimates of recovery efficiencies to generate rate vs.time decline curves.This is then compared with actual production to determine drainage area.Three basicquestions are then addressed: 1.Is the drainage area reasonables.2.Is the recovery assumption reasonables.3.Does the petrophysical analysis need adjustments.Additional adjustments might then be necessary.Results can be interpreted as follows: 4.If the petrophysical analysis suggests the well is under-performing,reservoir quality has degradedwithin the drainage area.5.If the petrophysical analysis suggests the well is over-performing,reservoir quality is better thanpredicted.This could be a consequence of contribution from a fracture system and/or from adjacentreservoirs close to the perforated interval.Results/Objectives/Conclusions: Results from the analyses of four oil wells in the Big Horn Basin ofWyoming are presented.All four wells have excellent log suites allowing for reliable petrophysical analysis,are now depleted,and have rate-time data available.The nominal well spacing is for 15-acre drainage.Petrophysical predictions for rate-time performancewere run for two basic cases: 1.For only the perforated intervals-often four of five within the gross perforated interval.2.For the gross perforated interval-petrophysical analyses indicated additional pay that was notperforated.Using a constant recovery efficiency of 20% the drainage areas were adjusted to get an exact match withactual performance.For three of the four wells,adjusted drainage areas are significantly greater than 15 acres.Factors influencing the calculated drainage areas are: 1.There is no well interference and drainage is occurring from adjacent undrilled acreage.2.Recovery efficiencies are higher than 20% 3.Oil is coming from zones additional to the gross perforated interval,or from a fracture system.Novel/Additive Information: The method compares petrophysical estimates of recoverable hydrocarbonswith actual rate-time performance,allowing for considerations of drainage areas and recovery efficienciesand the possibility of production coming from a fracture system and/or adjacent unperforated intervals.Estimates can be made from the petrophysical analysis of adjacent undrilled locations of rate-timeperformance before the well is completed.