Abstract
Abstract
The Valhall field, located in the Norwegian Sector of the North Sea, is a high porosity chalk field characterized by its weak reservoir rock. Global field behavior during depletion includes extensive compaction and associated subsidence.
One highly successful completion alternative, horizontal wells, was, in its early application to Valhall, plagued by the premature failure of the production tubulars associated with reservoir draw down/compaction. Numerical modeling, with subsequent field implementation, has shown, however, that casing with sufficiently low diameter:thicknesses can withstand the rigors of draw down and associated formation loads. Unfortunately, the use of thick-walled casing presents its own difficulties in terms of installing the string (torque/drag) and the subsequent limitations on tool diameter (smart well completions, for example) associated with the stout cross section. Further, a one-size-fits-all doctrine is not optimal, in that the porosity, and therefore strength, of the reservoir rock varies laterally, thus penalizing well designs on the flanks of the reservoir where the chalk porosity is lower and chalk strength is, therefore, higher.
The current paper addresses the above issues by modeling the behavior of the wellbore and surrounding region. Starting with the virgin reservoir, the model considers (sometimes extensive) pre-wellbore depletion, drilling the wellbore, installation of casing and cement, and subsequent draw down. Significant variations to the model include the cross section of the production casing, the quality and quantity of the cement sheath and the perforation pattern. Behavior of these various configurations during subsequent draw down permits them to be ranked according to the life expectancy of the resulting completion. The discussion is enhanced by field results, as several of the completion alternatives have been installed in Valhall wellbores.
Introduction
In an attempt to increase both the quantity and rate of recovery of hydrocarbons from the Valhall field, horizontal wellbores were introduced on March 22, 1991, with the completion of Well 2/8-A-12BSt1. Since that time a number of horizontal wellbores have been drilled, almost exclusively on the flanks of the field initially, but later also in the crest of the field.
Not surprisingly, a number of the initial horizontal completions, including cemented and uncemented liners and concentric liner configurations, failed or otherwise proved unsatisfactory. The Valhall producing environment is severe, the formation chalk undergoing extreme compaction during depletion1–3, with associated consequences for wellbores and tubulars penetrating the reservoir. At the other extreme, installation of unduly thick tubulars can result in difficulties both in running the string (e.g. torque, drag) and with regard to clearance restrictions in the implementation of smart completions.
The current study addresses the integrity of Valhall horizontal wellbores via a number of two-dimensional finite element simulations intended to model the entire history of the reservoir from discovery, through global pore pressure depletion, to the introduction of a wellbore, and, finally, through local production draw down. Accurate modeling is hampered by the extreme deformation that may occur prior to the introduction of the wellbore and by the uncertainty of cement coverage over long (1–2000 meter) horizontal sections.
The discussion begins with a description of the analysis variables, including reservoir and borehole conditions and the character of the casing, cement and formation chalk. A detailed description of the numerical modeling methodology is included to aid readers who may want to duplicate this analysis. These preliminary topics are followed by results for a number of completion, depletion and cementing scenarios.