The main objective is to build on prior publications of decline curve analysis, based on theoretical producing and fluid flow characteristics (Matthews and Lefkowits 1956, Fetkovitch 1975, Cox 1948).
In this paper, we relate petrophysical predictions of ultimate oil recovery to actual rate-time well performance by adjusting recovery efficiencies and well-bore drainage areas.
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 in other 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 basic questions are then addressed:
Is the drainage area reasonableš
Is the recovery assumption reasonableš
Does the petrophysical analysis need adjustmentš
Additional adjustments might then be necessary. Results can be interpreted as follows:
If the petrophysical analysis suggests the well is under-performing, reservoir quality has degraded within the drainage area.
If the petrophysical analysis suggests the well is over-performing, reservoir quality is better than predicted. This could be a consequence of contribution from a fracture system and/or from adjacent reservoirs close to the perforated interval.
Results from the analyses of four oil wells in the Big Horn Basin of Wyoming 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 performance were run for two basic cases:
For only the perforated intervals - often four of five within the gross perforated interval.
For the gross perforated interval - petrophysical analyses indicated additional pay that was not perforated.
Using a constant recovery efficiency of 20% the drainage areas were adjusted to get an exact match with actual performance.
For three of the four wells, adjusted drainage areas are significantly greater than 15 acres.
Factors influencing the calculated drainage areas are:
There is no well interference and drainage is occurring from adjacent undrilled acreage.
Recovery efficiencies are higher than 20%
Oil is coming from zones additional to the gross perforated interval, or from a fracture system.
The method compares petrophysical estimates of recoverable hydrocarbons with actual rate-time performance, allowing for considerations of drainage areas and recovery efficiencies and 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-time performance before the well is completed.