Reservoir evaluation of the shallow gas production areas in southern Alberta and southwest Saskatchewan has been accomplished by various methods including open hole and cased hole logs. The results after completion have been mixed: unexpected liquid mist production with the gas, less gas production than expected, or no production of any fluids. This paper will outline the geological nature of the Milk River reservoirs to help explain why the wells produce as they do. Production concerns and the current solutions to these concerns will be illustrated.
From these geological and production discussions will come an extension of Pulsed Neutron Capture technology to evaluate mineralogy and fluids. It will be shown how this technology can be utilized to indicate when unwanted water production could result after completion. Current studies will be shown where, under controlled conditions, this same technology can illustrate the rate of gas production anticipated.
Gas has been produced from the Milk River formation of southern Alberta and Saskatchewan since the beginning of this century. Commercial gas production has only been recognized in the past thirty years due to price increases. With the increased interest along with advancements in technology, new methods of evaluating these shallow low pressure gas reservoirs are continually being developed. This has enabled operators to improve production from existing wells and to maximize production from new wells.
Wireline logs used to evaluate the Milk River formation about thirty years ago consisted of an open hole resistivity log combined with a natural gamma-ray measurement (Figure 1). These logs assisted in identifying the thicker sand deposits but lacked the resolution necessary to identify the thinner sands. The resolution of resistivity also suffers here due to the fresh formation waters present, lower gas saturations, as well as the sequence of thinly bedded sands and silts. This lower resolution makes it very difficult to differentiate between a potential gas reservoir, water reservoir or tight zones. The thinly bedded inter-reservoir shales present throughout the salty sand deposits also mask the gamma-ray measurement, making reservoir identification with these logs difficult. Once major gas bearing zones were identified a cased hole gamma neutron log was the tool of choice to select completion intervals. This worked well for perforation selection but not for gas volume identification.
In the 1960's came the development of the dual detector neutron and the compensated formation density tools run in combination to enhance open hole evaluation. These tools along with the resistivity and natural gamma-ray measurements helped to better identify these gas prone sands. Using these logs today, the experienced log analyst can use a series of cutoffs to find the gas producing sands and avoid perforating low gas saturated, wet sands. Also tight zones, zones that can be sands cemented with carbonates or iron rich minerals, can be avoided.
The lower economic nature of these wells lends itself to multi-well projects drilled with an assembly line efficiency. The increased costs associated with the open hole logging, including the additional rig time involved, works against this operational efficiency.