The Expanding Role of the Drill Bit in Shaping the Subsurface
Authors: Saleri N.G. , Salamy S.P. , Al-Otaibi S.S. 
Published: June 2014

Distinguished Author Series articles are general, descriptive representations that summarize the state of the art in an area of technology by describing recent developments for readers who are not specialists in the topics discussed. Written by individuals recognized to be experts in the area, these articles provide key references to more definitive work and present specific details only to illustrate the technology. Purpose: to inform the general readership of recent advances in various areas of petroleum engineering.

Introduction

Technology and change are the operative words in the upstream industry. As advances across a wide band of disciplines (e.g., drilling, telemetry, and formation evaluation) continue to stream forward, the question remains, &#x0022;Is the emerging subsurface that much different from in the past?&#x0022; The confluence of three recent events - maximum reservoir contact (MRC) wells, geosteering, and increasing downhole intelligence - is creating a distinctively different subsurface environment. The intention of this work is not so much to predict the future face of the subsurface as it is to reflect on what has already happened. What was considered extraordinary (e.g., fish-bone wells, tens of thousands of feet of reservoir contact, live link with the drill bit) a few years back has now become routine. This article relays recent examples from various Saudi Arabian fields. A common thread in these examples is a large subsurface footprint coupled with more effective well placement by use of geosteering. Demonstrated results show remarkable gains in well productivities as well as reductions in unit development costs across a broad spectrum of fields.

MRC Wells: What Is so Different?

MRC wells are defined as having an aggregate reservoir contact in excess of 16,000 ft, through a single or multilateral configuration1 (Fig. 1). In February 2000, Saleri2 proposed the concept of targeting quantum leaps in reservoir contact as a routine and viable option for tight facies. A complete discussion on MRC wells, encompassing operational and reservoir-characterization aspects, is addressed by the same author1 and his colleagues, and forms the foundation of this article. Various studies elaborated on the trend toward increasing reservoir contact to achieve enhanced well productivities.3&#x2013;6

MRC wells represent a differentiation from conventional horizontal or multilateral completions, in terms of focus, intent, and architecture. First, the well focus is the intrareservoir segment (hence, the symbolic significance of the letter R in MRC); whereas the well trajectory from surface to total depth is the key defining attribute of extended-reach wells (such as those reported for Wytch Farm7), an MRC well is specifically keyed to the extent of the contact length within the target formation. Second, the intent is to achieve multiple-fold enhancement in well productivity, compared with established norms for respective fields. Third, the well geometry is governed by the reservoir-contact objectives, with no preconditions regarding architectural configuration (e.g., shape or completion type).

Fig. 2 shows schematics of four recent MRC wells from three carbonate fields, underscoring the diversity of new well geometries (i.e., fork, fish bone, and hybrid). All four wells were drilled and are on production. The desirable MRC configurations are case specific. Numerous considerations must be weighed - most notably, drilling operations, well control, hole stability, formation damage, monitoring, reliability, interference with offset wells, and comparative economics with alternatives.

While the selected well configurations (geometry, scale, and completion type) were based on considerable predrilling engineering analysis by multidisciplinary teams, they can be regarded best as products of necessary compromises among various factors, with maximum productivity and overall system reliability (hence, simplicity) being the dominant considerations.

Historical Background

The first application of MRC wells in Saudi Arabia occurred during 2002 in the Shaybah field. The field, discovered in 1968, contains Arabian Extra Light crude with an average gravity of 42&#x00B0;API and a solution gas/oil ratio (GOR) of 750 scf/STB. The field was developed in the mid-1990s with 3,280-ft horizontal wells. The presence of a large overlying gas cap and underlying aquifer necessitated the use of horizontal completions to mitigate gas encroachment while achieving desirable production rates in a tight-facies formation where typical permeabilities ranged from 10 to 40 md. The Shaybah field was placed on production in July 1998. Beginning in 2000, progressively increasing reservoir contacts were achieved in Shaybah producers in an attempt to maximize well productivities. The experience gained in Shaybah through MRC wells was also applied to other carbonate fields including Abqaiq and Ghawar.

MRC Cost and Time Breakdown

MRC wells differ from conventional horizontal wells in terms of both cost and time, as illustrated by Figs. 3 and 4 for two Shaybah wells at varying reservoir contacts. For drilling times, the intrareservoir portion increases from 29&#x0025; in a 3,280-ft horizontal (single lateral) to 74&#x0025; for an MRC well (fork trilateral). The unit-cost makeup shows a similar shift, with intrareservoir unit costs accounting for 62&#x0025; of the total (a manifestation of the increased focus on the target formation in MRC wells at the expense of the extra reservoir segment of the wellbore). In other words, higher productivities are achieved through higher expenditures in the target formation on a per-drilling-dollar-spent basis.

Completion Issues and Downhole Intelligence

In Shaybah, where the first MRC applications were used, reliability and operational-simplicity considerations were favored in the designs, whereby most of the reservoir contact was achieved through openhole completions (Fig. 2). For Well SHYB-753, a 1,000-ft liner was set at the entry of the target formation, and it served as the main platform from which the laterals and the remainder of the motherbore were drilled openhole. This hybrid design achieved multiple objectives: MRC, operational simplicity, and future flexibility for installing smart downhole completions.