Application of Short-Radius Horizontal Boreholes in the Naturally Fractured Yates Field
- J.R. Gilman (Marathon Oil Co.) | J.L. Bowzer (Marathon Oil Co.) | B.W. Rothkopf (Marathon Oil Co.)
- Document ID
- Society of Petroleum Engineers
- SPE Reservoir Engineering
- Publication Date
- February 1995
- Document Type
- Journal Paper
- 10 - 15
- 1995. Society of Petroleum Engineers
- 2 in the last 30 days
- 266 since 2007
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This paper discusses the performance and simulation of short radiushorizontal boreholes which are being used in the Marathon operated Yates FieldUnit in West Texas to minimize drawdown and therefore reduce gas and waterconing in a thin oil column. Yates is a very prolific field with extensivefracturing and high quality reservoir rock. Superimposed on a high densityorthogonal fracture network are widely spaced regional joints with a strongdirectional tendency. A major question is how these directional joints affectthe horizontal well performance and whether wells should be drilled parallel orperpendicular to the joints. Dual-permeability reservoir simulation studieswere performed to study optimum orientation of the borehole with respect to thenatural fracture network. Additionally, optimum well completion elevation wasstudied. Forty-six vertical wells have been recompleted as short radiushorizontal boreholes since 1986. The performance of the horizontal boreholeshas been outstanding. The 46 boreholes comprise less than 4% of the Unit'sproducing wells but they produce about 14% of the Unit production. The largeproductivity increase of the horizontal boreholes compared to the previousvertical completions have indicated that the wells are intersecting theregional joints.
GEOLOGIC DESCRIPTION AND RESERVOIR PERFORMANCE OF THE YATES FIELD UNIT
The Yates Field Unit is located 90 miles south of Midland, Texas at thesouthern tip of the Central Basin Platform (Fig. 1). Cumulative oil productionsince discovery in 1926 is 1.2 billion STB and the current oil rate (Dec. 1993)is approximately 50,000 STB/day from about 1000 active producing wells. TheYates Field Unit structure is an asymmetric, horseshoe-shaped anticline withgreater than 400 ft of closure that covers 26,400 acres. The major productiveinterval, the San Andres formation, is a highly fractured dolomite with matrixporosities as high as 35%. Two major fracture systems are present. Superimposedon a high density orthogonal fracture network are widely spaced regional jointsrelated to the structure flexure trends. The intense fracturing results inhighly productive completions, with initial well productivity indexmeasurements frequently exceeding 50 STB/day per psi of drawdown.
Gravity drainage is the dominant production drive mechanism with the largedensity difference between the oil and gas, and high quality matrix rockresulting in an efficient recovery mechanism. Although a gas cap was notpresent at discovery, a large secondary gas cap developed and has expanded overtime. The expanding gas cap at the top of the structure along with limitedaquifer influx at the bottom, have led to a thin oil column approximately 50 ftthick. The gas-oil contact is at a subsea elevation of + 1130 ft and thewater-oil contact at about +1080 ft.
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