Development and Application of PDC Core Bits for Down-hole Low Invasion Coring in the Arab Carbonates
- J.J. Rathmell (ARCO Exploration and Production Technology) | Bonsall S. Wilston (ARCO Exploration and Production Technology) | Bradley A. Gale (ARCO International Oil Gas Company) | David A. Bell (ARCO International Oil Gas Company) | Gordon A. Tibbitts (Baker-Hughes INTEQ) | Hughes Christensen (Baker-Hughes INTEQ) | Douglas J. Bobrosky (Baker-Hughes INTEQ)
- Document ID
- Society of Petroleum Engineers
- SPE Drilling & Completion
- Publication Date
- March 1998
- Document Type
- Journal Paper
- 56 - 65
- 1998. Society of Petroleum Engineers
- 2.2.2 Perforating, 1.6.9 Coring, Fishing, 1.10.1 Drill string components and drilling tools (tubulars, jars, subs, stabilisers, reamers, etc), 1.8 Formation Damage, 1.1 Well Planning, 6.5.4 Naturally Occurring Radioactive Materials, 1.11 Drilling Fluids and Materials, 5.2 Reservoir Fluid Dynamics, 5.5.11 Formation Testing (e.g., Wireline, LWD), 1.2.5 Drilling vibration management, 1.6.1 Drilling Operation Management, 5.1.1 Exploration, Development, Structural Geology, 1.6 Drilling Operations, 5.5.2 Core Analysis, 5.6.5 Tracers, 3.4.5 Bacterial Contamination and Control, 1.5.1 Bit Design, 5.4.10 Microbial Methods, 5.6.4 Drillstem/Well Testing, 1.10 Drilling Equipment, 1.11.2 Drilling Fluid Selection and Formulation (Chemistry, Properties), 1.2.3 Rock properties, 5.6.2 Core Analysis, 5.6.1 Open hole/cased hole log analysis, 4.3.4 Scale
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A method has been developed for low invasion, down-hole motor coring in the Arab formation, Arabian Gulf. Motors were used for the first time in the Arab formation to achieve higher bit rotational velocity and higher coring rate for low filtrate invasion when using a water base mud.
Earlier, a field test of motor coring with pre-existing low invasion core bits showed antiwhirl core bits were superior to other core bits at high bit rotation rates. Two new anti-whirl, low invasion core bits with improved PDC cutters were designed and lab tested for coring the Arab carbonates and anhydrites Lab coring tests indicated a two-fold increase in coring rate for the new core bits with a motor compared to when using only the top drive.
The low invasion waterbase coring mud and core bits tested in the laboratory have been used to cut 278 feet of core on the Al-Rayyan field in the Qatari section of the Arabian Gulf. A down-hole motor and top drive provided rotational rates at the core bit exceeding 200 rpm compared to 100 rpm with top drive alone. Motor coring has provided a two-fold coring rate increase. Core quality was excellent, core recovery was 99% with low filtrate invasion confirmed by potassium iodide tracer. Well log and capillary pressure derived water saturations agree with the core.
The Al-Rayyan field is located approximately 45 miles north-northeast of the Qatar peninsula, as shown in Figure 1, where water depth is 100 feet. The field is in the northernmost portion of the ARCO Consortium's Eastern Area. Initial drilling on the structure began in 1976 by an international consortium. The discovery well located oil columns in the Arab A and Arab C zones. The Arab C zone flowed oil at rates up to 1300 bbl/d with a high skin factor. A combined test of the Arab A and B zones produced 1500 bpd of a tight oil-water emulsion. The discovery well was followed by two delineation wells which were drilled in 1976 and 1980, respectively. Although both delineation wells found an oil column, the accumulation was deemed non-commercial.
In late-1995, the ARCO consortium drilled a fourth well on the structure with the objectives of (1) understanding water saturation distribution in the Arab A and Arab C reservoirs, (2) obtaining native state core material for special core analysis programs, (3) demonstrating the ability to drill undamaged wells, and (4) determining the productivity of both vertical and horizontal completions. Well 4 showed a commercial oil accumulation existed in the Al-Rayyan field. The Arab A and Arab C zones were cored, logged, and tested in a vertical pilot hole. The low invasion core played a critical role in understanding the reservoirs properties. A horizontal well through Arab C zone produced at rates in excess of 9600 bbl/d in short-term, horizontal well flow tests.
Low invasion coring has become a standard approach for, ARCO in domestic and international well locations. Successful low invasion coring in the well 4 involved the following equipment and procedures:
1. Two new parabolic, low invasion PDC core bits having premium cutters (polished, concentric ring reinforced) and antiwhirl technology, were designed and lab tested for motor coring.
2. Higher bit rotational rate was obtained with a down-hole motor to maximize coring rate.
3. A coring mud with low static fluid-loss and low spurt velocity was used. Full-scale lab coring tests were run with the mud before it was used in the field. This mud also minimizes formation damage.
4. A logistical plan was developed to obtain core plugs within 30 to 48 hours at a contract lab in Abu Dhabi.
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