A New Approach To Fracturing The Milk River Formation In Southeastern Alberta

Abstract

Abstract The shallow Milk River gas sands of southeastern Alberta contain significant gas reserves. Recent improvements in production logging techniques and greater reservoir and completion knowledge of the Milk River zone indicated that current fracturing techniques were less than adequate to develop the potential of this zone. A series of wells were completed using conventional Milk River fracture treatments and then logged to determine the effectiveness of the fracture treatment on the Milk River zone. The results indicated that only one or two sets of perforations were being stimulated. On the basis of the above information, a stage fracturing technique was designe1t in which the Milk River zone would be fractured in two separate treatments. The fracture treatments were also redesigned to complement the new fracturing technique. Two separate Medicine Hat/Milk River shallow gas projects were chosen to evaluate the effectiveness of the stage fracturing concept and the redesigned fracture treatments. The initial after fracture results indicated a 91% and 130% improvement in productivity from the Milk River zone, for the respective projects. The results obtained to date validate the concept of stage fracturing the Milk River zone. The initial average increased gas production from the Milk River zone of 6 100 m3/d (STD) and its related $20 000 cost has a payout period of approximately 43 days per well. Long term production tests are being undertaken to determine if there is a sustained level of increased production between the wells recently completed and wells previously completed using the old fracturing techniques. Introduction Throughout the past ten years there has been considerable ctivity in the southeastern portion of Alberta, one of the main zones of interest being the gas bearing Milk River formation. Figure I indicates the location of the Milk River formation in Alberta. The Milk River formation is found at relatively shallow depths and hence is often referred to as shallow gas. The Milk River formation has a gross thickness of approximately 110m. The formation is an extremely heterogeneous mixture of coarse grained siltstone varying to fine sandstone irregularly distributed through a dark to medium, grey, soft, silty shale. The siltstones have a high clay content, the predominant constituents being the Kaolinite and Illite clays. The clays often account for as much as 10% of the bulk volume. The average permeability values of the Milk River formation are less than 0.1 md. Hydraulic fracturing is required to increase productivity due to the low permeability thickness of the reservoir and the low reservoir pressure. Identification of potential zones on logs has been hampered by the fact that connate water in the Milk River is fresh and prevents the differentiation of gas from water using a standard resistivity log. The producible sand lenses in this reservoir have been difficult to detect using conventional log analysis due to the shaliness of the reservoir. In the last ten years, better logging tools such as the Dual Spacing Thermal Neutron Decay Time Log and the Neutron Lifetime Log, both previously Described(l) (2), have been developed to better define potential zones.