New Electromagnetic Surveying/Ranging Method for Drilling Parallel Horizontal Twin Wells

Abstract

Summary An electromagnetic source was used in conjunction with a measurement-while-drilling (MWD) survey instrument to drill two parallel horizontal wells with a vertical separation of 9±2 m over a length of 600 m in Amoco's Wolf Lake field for steam-assisted gravity drainage of heavy oil. Existing wellbore survey methods are incapable of this level of precision because of cumulative survey error and associated position uncertainty. Using this new electromagnetic ranging technique, the twin wells were placed with better than 10 times the accuracy of gyro surveys. Background The steam-assisted gravity drainage (SAGD) process involves placing a horizontal production well directly beneath a horizontal injection well so that the wells are in alignment along their entire wellbore lengths. A steam chamber is formed around the upper steam injection well, and condensed steam (water) and oil flow down along the edge of the chamber to the lower production well, as shown in Fig. 1. The SAGD process was chosen for Amoco Wolf Lake because of the high viscosity (100,000 cp) and low mobility (10°API) of the crude oil and the homogeneity of the reservoir. An existing cyclic steam stimulation project on 1.6 hectare spacing has indicated that recoveries of only 18% are achievable with that method of production. With SAGD, modeling has indicated recoveries of up to 60%. In June 1993, Amoco drilled a pair of SAGD horizontal wells at Wolf Lake, the HWP1L (lower) producer and the HWP1U (upper) steam-injection well, spaced vertically 9 m apart. The wells were surveyed with MWD, gyro, and electronic multi shot. The upper well was drilled second and guided with a modified MWD tool. Distance and direction were measured precisely with an electromagnetic source deployed in the lower well. SAGD has been under investigation by the Alberta Oil Sands Technology and Research Authority (AOSTRA) since 1987. The process has been tested with two 50-m well pairs1 and three 500-m well pairs drilled horizontally from the AOSTRA Underground Test Facility. The 500-m pairs are projected to produce 650 B/D each for their full economic life of 4 to 5 years.2 Critical to the process is precise well separation through the horizontal injection/production interval. Separation requirements vary from 4 to 10 m, depending upon reservoir characteristics, with tolerances of no more than ±2 m and the injector directly above the producer. Performance of SAGD pairs is maximized by precise control of well separation along the entire horizontal section to fully initialize the process and avoid unwanted steam breakthrough.3 For SAGD well pairs drilled from the surface, the stringent separation requirements cannot readily be met by conventional wellbore surveying tools and methods. The position uncertainty of horizontal wells drilled and surveyed using available steering tools, MWD's, or gyros exceeds the SAGD tolerances and can exceed the separation requirements themselves. The problem is further compounded for magnetic tools by interference because the second well is drilled in close proximity to the steel casing or liner of the first. Clearly a method was needed to drill precisely separated well pairs that did not depend upon wellbore surveys tied to a surface coordinate system. The MGT allows the second well of an SAGD pair to be precisely drilled with respect to the actual position of the first well. Steering is relative to the MGT electromagnetic source in the first well, eliminating accumulated survey errors from both wells. Position Uncertainty Wellbore survey error and associated position uncertainty have been studied in detail, most notably by Wolff and de Wardt,4 who determined the errors to be largely systematic. Slight errors in survey inclination of the magnitude expected with good survey quality control (0.1° to 0.2°) will cause a few meters of true vertical depth (TVD) error in typical horizontal wells. Other contributors to TVD error are depth measurement errors in either the drillpipe tally or wireline and depth datum errors. Azimuth error and lateral uncertainty in horizontal wells are primarily caused by compass error. For magnetic instruments and gyros with good quality control, precision of 1° is obtainable. Magnetic compass error and lateral uncertainty are greatest in horizontal wells with east-west orientations and at high latitudes where the horizontal component of the earth's magnetic field is small. For a typical SAGD well profile, 1° of azimuth error translates into 3 to 5 m of lateral displacement at the end of the build and to over 10 m at the end of a 500-m horizontal section. At the onset of the Wolf Lake SAGD project planning stage, it became apparent that conventional wellbore survey methods would not provide the accuracy required by Amoco for each wellbore (± 1.0 m vertically and ± 2.0 m laterally). Three survey tools were evaluated to determine what the best possible accuracy could be for each of them under optimum conditions. The survey tools evaluated were MWD, electronic magnetic multishot (EMS), and North-seeking gyro. These tools and magnetic ranging to the MGT are compared in Table 1. An inertial navigation system was not considered because of cost, availability, and limited hole size. MGT Tool Principles The MGT tool is an electromagnetic field source deployed in a first well and sensed by a modified MWD in a second well. When the tool is energized, a magnetic field of known strength and orientation superimposed on the local magnetic field. The strength and orientation of this field is used to compute distance and direction between the MGT source in the first well and the MWD sensors in the second. As shown in Fig. 2, magnetic field lines generated by the MGT tool lie in the plane defined by the tool and the magnetic sensors of the MWD. The direction of the radial component of the field relative to the borehole highside direction (indicated by accelerometers in the MWD) determines the angle to the MGT tool and the first well. The strength of the radial and axial fields measured at the MWD is used to compute the separation between the axis of the MGT tool and the MWD sensors as well as the depth difference between the MWD and the MGT. To verify the accuracy of the MWD/MGT system, measurements were taken on the surface with the MGT in 5.5-in., 21-lbm/ft casing on a line parallel to the casing at a typical SAGD separation (Table 2).

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