DRILLING & PRODUCTION
Leonard LeBlanc Houston Small earthquakes induced accidentally in producing zones in the northern and eastern North Sea are triggering increases in production through re-pressurization of reservoirs. In other cases, field production has partially collapsed, shutting off production entirely. The process is well known in mature basins in the US, but such events, some induced deliberately, are increasing in the North Sea.
Induced earthquakes boosting output in North Sea wells
Small earthquakes induced accidentally in producing zones in the northern and eastern North Sea are triggering increases in production through re-pressurization of reservoirs. In other cases, field production has partially collapsed, shutting off production entirely. The process is well known in mature basins in the US, but such events, some induced deliberately, are increasing in the North Sea.
Researchers at Heriot-Watt University and Edinburgh University are studying the process in order to develop ideal producing zones for earthquake induction with the objective of increasing production. As fields mature in the North Sea, seawater injection is being used to enhance the remaining productive life of the reservoir. In some cases, particularly those where earthquakes have been recorded, the results have far outweighed expectations. Researchers say the rock matrix in the reservoir and over burden is buffeted by two forces:
- The evacuation of pore fluid from the rock spaces changes the reservoir pressure and physically stresses the rocks.
- The injection of seawater introduces a corrosive environment, which dissolves and degrades the strength of the weaker contact points in the rock matrix.
As a result, substantial portions of the reservoir and over-burden can collapse, re-pressurizing the reservoir. The period between injection and widespread rupture can be days or weeks.
The earthquakes experienced thus far measure 4.0 on the Richter scale, and are largely confined to the immediate area. The northern North Sea is an active seismic zone. The two universities have developed a computer simulation program that can spot such reservoirs that are ideal for earthquake-induced production enhancement.
GRI well fracturing study to map treatment, results
About 20,000 well fracture treatments are performed each year, largely in the US, with an estimated cost of about $860 million. Well fracturing works, sometimes spectacularly, but the linkage between treatment and results is not very well understood.
The US-based Gas Research Institute hopes to develop a greater understanding, sufficient to create a commercial diagnostic service. The research focuses on a field in Colorado where the fracturing process will be studied in detail from the wellbore and surface, and comparisons with simulated models will take place. Some commercial systems exist now, and GRI will use those to develop historical profiles of reservoir rock structure, treatment, and results for matching purposes.
What GRI will do in Colorado is to map the actual extent of fractures, something that has not been done previously. Estimates are made now on fracture length, conductivity, azimuth, and height from models, and the results used to calculate fracture pressures and timing. Follow-up investigation is rare. Wellbore vertical seismic profiling can map the fractures, but the results cannot be produced within a reasonable time frame for well completion purposes.
GRI envisions the placement of specially developed borehole seismic receivers, which would record events during hydraulic fracturing and afterward. The shear fracturing process creates compressive (p) and shear (s) waves of seismic energy. The seismic receivers would calculate the distance to the fractures by the arrival time of the separate waves, and provide location, azimuth, and configuration of the fracture.
REFERENCE: Warpinski, N., "M-Site Results move Industry closer to Real-Time Hydraulic Fracture Mapping," GasTips, Gas Research Institute, Fall, 1995.
The effort to use deep draft hulls to reduce response to topside conditions continues with the Technip Geoproduction deepwater production design. The raft is jacked down from the catenary-moored semisubmersible floater's watertight hull and columns to gain stability, which allows the wellheads to be positioned on deck. The unit has storage capability and can produce in up 3,300 ft.
Trinidad well reworked with thru-tubing CT and no drilling unit
A well off Trinidad that sanded up after acid stimulation was successfully recovered after being cleaned out, re-completed, and gravel packed through the tubing string with a coiled tubing unit and no rig. The AmocoTrinidad well, Teak E-11, is among a select few wells that have been recovered after significant sanding with minimal surface equipment.
Originally, the well had been gravel packed around slimpak screens across 338 ft of open hole in 1993. Well flow, which started at 500 b/d, was down to 140 b/d with 10% water before acid stimulation and sanding up. Flow after completion and gravel packing was 308 b/d with 13% water. The cost of the operation was US$200,000, compared with $800,000 for a conventional sand cleanout and gravel packing. A short report on the well appeared in the November issue of The Brief.
Extended reach record pushed to 8 km in UK
The Wytch Farm field in the UK features drilling from an island into a reservoir under Poole Bay, just off the coast of southern England. The field has produced numerous records, the latest of which includes an 8 km extended reach well. BP drilled the M05 well a stepout distance of 8,035 meters, with a true vertical depth of 1,611 meters. The well took 115 days to drill and case. Anadrill was the directional and MWD contractor for the well.
Wet welding takes major step with better electrode
A research project to reduce drydocking of a large number of aging US Navy ships has developed a by new underwater welding electrode that overcomes most of the problems associated with wet welding. The new electrode, which is black in appearance, requires no mechanical shielding.
An Ohio State University program, operating under US Navy/National Sea Grant funding, has developed the electrode. A two-year test of the electrode found that weld appearance was excellent and micro-cracking was eliminated. The inventor is Dr. Chon Tsai.
Two additional benefits provided by the new electrode are that it provides good metal deposition in virtually any position and the electrodes are less expensive than conventional electrodes. The cost of the new electrodes will be $7-10/lb, versus the $13-35/lb of conventional electrodes. The electrode will be manufactured by Welding Consultants (Tel: 614-258-7018).
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