Horizontal Core Acquisition and Orientation for Formation Evaluation
- R.A. Skopec (Oryx Energy Co.) | M.M. Mann (Diamant Boart Stratabit) | Doug Jeffers (Scientific Drilling Intl.) | S.P. Grier (Core Laboratories)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling Engineering
- Publication Date
- March 1992
- Document Type
- Journal Paper
- 47 - 54
- 1992. Society of Petroleum Engineers
- 1.6.6 Directional Drilling, 5.6.1 Open hole/cased hole log analysis, 1.10 Drilling Equipment, 5.5.2 Core Analysis, 1.6 Drilling Operations, 5.8.2 Shale Gas, 3 Production and Well Operations, 1.11 Drilling Fluids and Materials, 5.6.2 Core Analysis, 5.1 Reservoir Characterisation, 1.12.1 Measurement While Drilling, 5.3.9 Steam Assisted Gravity Drainage, 1.6.1 Drilling Operation Management, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 5.8.7 Carbonate Reservoir, 1.12.6 Drilling Data Management and Standards, 1.6.9 Coring, Fishing
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The increase in horizontal drilling activity has produced a need for improved coring technology. The development of a reliable horizontal (medium-radius) coring and orientation system has greatly improved the acquisition of information necessary for formation evaluation and reservoir engineering. This paper describes newly developed hardware and methods for obtaining horizontal core sections.
Oryx Energy Co. has successfully recovered horizontal core from highly fractured carbonate rocks in south and west Texas. Acquisition of representative reservoir rock during drilling is the most tangible technique for gaining direct knowledge of the subsurface. Data obtained from horizontal core are prerequisites for developing realistic projections of reservoir performance. Characterization of the fracture network geometry and the degree of connectivity is essential to understanding fluid flow. When a reservoir is being developed with horizontal wellbores, a considerable amount of geological information is needed to calculate its volumetric potential and quality. Production is optimized in fractured reservoirs when the horizontal wellbore intersects the fractures in a direction perpendicular to their surfaces. Lateral evolution of facies and fracture orientation is obtained most reliably from core material. Horizontal wells also can be highly effective in reservoirs where conventional wells have low productivity, where productivity is limited by water and gas coning, and in thick continuous sands containing heavy oil and bitumen where steam-assisted gravity drainage is possible.
Wireline logs cannot be used as a substitute for the information gained from core material. Conventional logs, such as the nuclear, electric, and sonic tools, can be run easily in horizontal wells; however, they exhibit very little character and are insensitive to fractures. Advances have been made in fracture delineation with electrical imaging and acoustic waveform logs. Difficulties, such as distortion of fracture azimuth/aperture and sensitivity to bore-hole rugosity, may arise with these devices in distinguishing open and mineralized fractures. In many instances, well conditions may prohibit well logging altogether, and the cost of running specially logs can be quite high. Comprehensive formation evaluation in horizontal wells is accomplished best through the integration of core and log analyses.
Few references to horizontal coring exist in the literature. Giger and Jourdan reported extraction of core from oil-bearing limestone reservoirs at Lacq, France; Casters Lou, France; and Rospo Mare in the Italian Adriatic Sea. Operators in the U.S. have been slow to adopt horizontal coring, and many insist on coring the formation vertically and re-entering the borehole for completion of the horizontal section. This method is problematic because reservoirs targeted for horizontal drilling.
In the past, the petroleum industry has suffered from poor coring performance, resulting in high costs of obtaining core from indurate and performance, resulting in high costs of obtaining core from indurate and fractured formations. 6 The primary reason for poor recovery was the frequent jamming of the rock in the - coring assembly. This problem has largely been overcome by the use of flexible fiberglass inner barrels and multiple core-catcher assemblies. On four horizontal coring projects, Oryx achieved a recovery rate of almost 100% by using shorter core sections and improved inner-and outer-barrel stabilization. For orientation purposes, a slip scribe is used in conjunction with an electronic multishot instrument (EMI). The EMI is a solid-state survey device that uses an array of accelerometers for determination of inclination. Magnetometers determine orientation with respect to geographic north. Many of the problems associated with camera surveys have been alleviated through the use of EMI technology. To operate the EMI in a horizontal configuration, it was necessary to develop specialized equipment. A newly designed rotatable nonmagnetic blade centralizer, enhanced running gear, and a high-frequency shock absorber must be used to obtain accurate core orientation data.
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