An Investigation Into The Pi Behavior Of Wells In The Yibal Shuaiba Chalk Reservoir, Oman
Authors: Johns R. , Lingen P.L. 
Published: December 2007

Oil wells in the Yibal Shuaiba chalk reservoir have been observed to show a wide range of initial PI values and the field performance has been characterised by the unexpectedly rapid decline of the PI values in the majority of the wells. Results obtained from a numerical simulation study based on an investigation of the performance of two 'type' wells show that this performance of two 'type' wells show that this PI behaviour is governed mainly by the degree of PI behaviour is governed mainly by the degree of (1) the secondary permeability present (leaching/ fissures) in the producing zones and (2) the permeability reduction in these zones due to a permeability reduction in these zones due to a probably inelastic compaction of the formation probably inelastic compaction of the formation accompanying the declining pore fluid pressure. Results from prediction studies are being viewed as a guide to field development planning.

Introduction

The Yibal Shuaiba reservoir is an anticlinal structure of some 50 sq km2 with a maximum oil column of 300 ft. The reservoir rock is fairly homogeneous with low matrix permeabilities (1 - 30 md range). Previous studies indicated the necessity of initiating an early water injection scheme to achieve pressure maintenance. Under such a proposed scheme a reasonably high recovery factor is expected, considering the homogeneous nature of the reservoir, the favourable mobility ratio (0.5), and the observed low residual oil and connate water saturations (0.20 and 0.05 respectively). Consequently the field has been developed with a 9-spot water injection pattern in the crestal area.

The field commenced production in 1969 with an initial reservoir pressure some 100 psi above the bubble point pressure of 2128 psia and water injection was commenced via 8 wells in 1972. In spite of this the reservoir pressure has been declining steadily and is at present +/- 150 Psi below the bubble point pressure. Consequently the producing GOR has increased to approximately twice the solution GOR.

The past production performances of many wells in the field have been characterised by rapid reductions of their initial PI values; initial values ranging from 4 to 20 B/D/psi falling to a fairly consistent, but still slowly declining, field wide average of +/- 1.5 B/D/psi after approximately 1 to 2 million barrels of cumulative oil production.

This PI decline has caused the present depletion rate of the reservoir to be rather low. To increase the offtake and to simultaneously halt the reservoir pressure decline a large extension of the production and injection facilities are at present in progress.

For the design of this development extension an accurate understanding of the average well PI behaviour is essential. As it was thought originally that the observed PI behaviour, in view of the increasing GOR, was caused by a gas blockage effect it was felt likely that the individual well PI values might increase substantially after pressure stabilisation.

It had also been recognised from laboratory measurements on core samples that the uppermost region of the Shuaiba may exhibit a significant inelastic compaction behaviour with declining pore fluid pressure. This irreversible pore fluid pressure. This irreversible compaction effect was likely to cause a permeability reduction which would contribute to a PI permeability reduction which would contribute to a PI decline.

To gain insight into the physical mechanisms causing the observed PI behaviour and to provide a sound basis for field development planning the numerical simulation study described in this paper was undertaken. paper was undertaken. Numerical models have been defined for two specific 'type' wells showing a significantly contrasting PI behaviour. The numerical simulator has been adapted to include a permeability pressure relationship, defined as a permeability pressure relationship, defined as a permeability reduction accompanying a reduction in the pore fluid pressure.