Hydraulic Fracturing of Horizontal Wells: An Update of Design and Execution Guidelines

Abstract

Hydraulic Fracturing of Horizontal Wells: An Update of Design and Execution Guidelines Olivier Lietard, SPE, Dowell Division of S.E.P.S. (U.K.) Ltd., Joseph Ayoub, SPE, Compagnie des Services Dowell Schlumberger and Anthony Pearson, Isaac Newton Institute, Cambridge University Abstract Thousands of horizontal wells have been drilled world-wide over the last ten years. Most of them are completed openhole and do not produce as well as expected. In their design phase, adverse reservoir and wellbore effects are often underestimated, such as permeability anisotropy well stand-off, formation damage, formation face damage, and sand control screen plugging. Recent statistical surveys of the production of openhole horizontal wells have confirmed that actual performances are disappointing. They are leading to major reevaluations of drilling and completion strategies in many present reservoir developments. In offshore environments where a limited number of slots are available, horizontal wells remain the best choice. However they are increasingly cased, cemented, perforated and stimulated, in order to alleviate the problems affecting openhole wells. This paper describes how this new trend is expanding in the North Sea, and gives all necessary guidelines to successfully design and execute multiple fracturing treatments in horizontal wells. Production and treatment cost estimates are provided for all cases of fracture orientation vs. the wellbore. Radial flow effects in noncollinear geometry are detailed and Tip Screen Out (TSO) designs recommended. Introduction The number of horizontal wells drilled world-wide so far has become statistically significant, particularly in North America. An increasing number of studies are published concerning the actual performance of openhole horizontal wells, as compared to expectations. The most detailed and famous paper is comparing reservoir development with horizontal wells to gambling, showing that actual Productivity Improvement Factors (PIF's) - as compared to vertical wells - most often equal 2, when equations would promise much larger values (easily in excess of 10 even in primary porosity reservoirs). Short articles have recently emphasised the disappointing outcome of the technology, which seems to give positive results in naturally fractured rocks only (e.g. the Austin chalk trend), whereas economical success in tight gas recovery is not achieved by half the horizontal wells. Causes of poor production of openhole horizontal wells are numerous and will be detailed in this paper. Reservoir development schemes have unfortunately too often disregarded these adverse effects. Overoptimistic production estimates are derived from simplistic equations where permeability anisotropy, well stand-off and skin values are optimistically defaulted. In an offshore environment where economical success is more difficult to achieve, poor actual production has a catastrophic impact, sometimes leading to the cancellation of field development programmes. This is particularly true in the North Sea, where harsher weather conditions and increasing water depths significantly affect the economics of a large number of new fields (particularly West of Shetland). As a consequence, acknowledgements of failure have come sooner than in any other part of the world (relatively to the lifetime of the technology) and countermeasures have rapidly been implemented, sometimes after preliminary onshore testing. This has led to the present scenario whereby all the high rate treatments (hydraulic fracturing or matrix) pumped in the first six months of 1996 by the three available stimulation vessels in the North Sea concern cased and perforated horizontal wells. P. 723