Multiple Proppant Fracturing of a Horizontal Wellbore: an integration of two technologies
Authors: Norris M.R. , Berntsen B.A. , Myhre P. , Winters W.J. 
Published: December 2007

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.

Introduction

The Valhall Field is an Upper Cretaceous, asymmetric chalk anticline which forms an overpressured, under saturated, oil reservoir located in the Norwegian sector of the North Sea. It is characterized by high porosity (25-48%) and high oil saturation (92-97%) and, in common with other chalk reservoirs in the region, the drive mechanism is a combination of fluid expansion and formation compaction. The production is mainly drawn from the high porosity (35-48%) Tor formation which exhibits a typical thickness of 80–100 ft and a permeability of 1–10 mD. In some parts of the field the underlaying, lower porosity (25-35%), Hod formation is also productive with a permeability of 0.5–2 mD. The source rock for this hydrocarbon deposit is the organically rich Kimmeridgian Clay underlaying the chalk. This has resulted in a reservoir which is over pressured for depth with a undepleted gradient of 0.80 psi/ft TVD at 8200 ft TVD, Ref 1.

The field originally held estimated reserves of 585 MMSTB and currently produces at 80,000 BOPD from 30 wells.

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