In the North Sea chalks, initial horizontal wells were blanket perforated and either matrix- or fracture-stimulated with acid. Within six months, chalk influx, lost productivity, and liner deformations were severe and caused lost access to most of the horizontal sections. Located at 8200 ft depth, the Tor and Hod formations of the Valhall Field have permeabilities ranging from 0.5 to 10 md. New horizontal wells were stimulated with 16/20 LWC and aggressive TSO designs, resulting in twice the productivity, and 150% the cash flow of conventional wells
Oil production from horizontal wellbores in the Valhall field has often proved both unstable and below expectation in this soft chalk formation. Multiple acid etched fractures from such wellbores have yielded encouraging results but exhibit limited longevity due to compaction. This paper describes the integration of proppant fracturing and horizontal well technology to provide a long term cost effective solution and so yield both high and sustained production in this field.
A numerical model was calibrated against known production profiles for both existing horizontal wells and conventional proppant fractured wells in the field. This was then configured to determine the production decline rates for various multiple proppant fractured horizontal completions operating under a standardized drawdown schedule. This indicated the number, size and conductivity of proppant fractures to provide an optimum risk weighted economic return. Convergent flow pressure drop was evaluated using a derivation of the Forcheimer Equation to allow radial flow to be analyzed.
Proppant pack conductivity testing was performed under specific reservoir conditions of placement, stress, temperature and flowback while also using an optimized carrier gel and breaker concentration. New resin coated ceramic proppant were benchmarked and a selection made with due consideration for conductivity and pack strength under convergent flow conditions. Using these results the required fracture conductivity was developed using tip screen out (TSO) methods to maximize propped fracture width and offset embedment into the soft chalk.
A specific methodology was followed for the perforating, fracturing, cleanout and isolation of each production zone. Individual fracture productivity has been determined by flowing the well for an extended period following each treatment. As expected, the productivity index doubled following the second treatment, however the third treatment yielded only a marginal increase in overall productivity. A number of possible causes have been investigated.
Benefits from integrating horizontal well and proppant fracturing technology are illustrated by the post completion economic analysis. This indicates, that despite the increased cost of the completion, the payout time is 15% sooner, with a rate of cash return 100% greater, than that of the average horizontal well in the field.
The process has now been further improved since this pilot project was placed on stream. Completion costs, and execution time, have reduced significantly through the use of large size coil tubing in a revised wellbore design. This has further allowed the completion phase, as described in this paper, to be performed as a concurrent activity entirely off rig time.
Author(s): M.R. Norris, Dowell Schlumberger; B.A. Berntsen, P. Myhre, W.J. Winters, Amoco Norway Oil Company
Paper Number: SPE 36899