Formation Analysis and Reservoir Performance StudyTensleep Reservoir, Hamilton Dome Field
Authors: Thompson J.K. 
Published: December 2007

Summary
A joint geologic/engineering study of the Tensleepreservoir in the Hamilton Dome field in Wyomingwas undertaken (1) to define reservoir characteristics, (2) to evaluate fully the reservoir's primaryperformance, and (3) to recommend operation andexploitation procedures to maximize oil recovery efficiency. An improved understanding of reservoirzonation, natural formation fracturing, and waterinflux behavior resulted in a development drilling program on several leases that significantly increasedoil production rate and ultimate oil recovery.Prediction methods also were developed to projectfuture water influx volumes and probable water/oilratio (WOR)trends.

Introduction
The Hamilton Dome field is on the southwest side ofthe Big Horn basin in northwest Wyoming (Fig, 1).Production has been obtained from six geologic strata ranging in depth from 1,500 to 3,500 ft (457 to1067 m) over a productive surface area of 2,500 acres(10 × 10(6) m). The structure, discovered in 1918, had500 million bbl (79 × 10(6) m3) of original oil in place (OOIP) in the six producing formations and hasproduced at rates of more than 25,000 BOPD (3975m3/d oil).
This paper principally deals with the major payhorizon in the field- the Pennsylvanian Tensleepreservoir - which had an OOIP volume of 375 millionbbl (60 × 10(6) m3). The Tensleep reservoir produces20 degrees API (0.93-g/cm3), 42-cp (0.042-Pas)crude from an average depth of 2,750 ft (838 m).Cumulative oil production totals 147 million bbl(23 × 10(6)m3) of oil with producing energy in thishighly undersaturated reservoir supplied by a veryactive water drive. Basic rock and fluid properties are summarized in Table 1 and are averages; actual fluidproperties vary significantly with structure asdiscussed in a later section.
The Tensleep formation, which averages 200 ft (61m) in net pay thickness, was considered, in previousreservoir studies, a relatively uniform, massive sandstone overlying an occasionally productive basaldolomitic section. However, more recent well logsfrom a few wells in the late 1960's and mid-1970's indicated a more complex zonation. Selective testingin some of the same wells showed higher pressuresand more economic water cuts in certain lower Tensleep intervals, indicative of poor drainage.
Also, water encroachment in the field wasextremely erratic. Numerous wells in the center of thefield had experienced water breakthrough and reached noneconomic water cuts, while wells locatedon the structure both downdip and updip of thecentral region were water-free or at low WOR's.A joint geologic/engineering study of the Tensleepreservoir was undertaken (1) to define reservoircharacteristics, (2) to evaluate fully the reservoir's primary performance, and (3) to recommend futureoperation and exploitation procedures formaximizing oil recovery efficiency.
An improved understanding of the Tensleep'scomplex reservoir zonation led to an infill drillingprogram, which significantly increased oil rate andultimate recovery. Shut-in of high water cutproducers in certain problem areas further boostedoil production.A concurrent analysis of infill drilling results andthe water influx behavior as influenced by naturalformation fracturing has helped to explain theunusual water encroachment pattern in the field. Thefracture study discussed in this paper applies previous laboratory and geology field work toillustrate natural fracture geometry on foldedstructures. Prediction methods also were developed to project future water influx volumes and probableWOR trends.
JPT
P. 245^