Integrated Approach To Optimize Material Selection for High-Rate Gas Wells
Authors: Bybee Karen 
Published: February 2015

This article, written by Assistant Technology Editor Karen Bybee, contains highlights of paper IPTC 12503, "Integrated Approach To Optimize Material Selection for North Field High-Rate Gas Wells," by W.A. Sorem, RasGas Company; E.J. Wright and J.L. Pacheco, ExxonMobil De vel opment Company; and D.A. Norman, ExxonMobil Upstream Research Company, originally prepared for the 2008 In ter national Petroleum Technology Conference, Kuala Lumpur, 3-5 December. The paper has not been peer reviewed.

The RasGas Company North field wells typically have a true vertical depth of 9,000 ft with a sail angle up to 70º. The combination of 7-in. mono-bore and 9 5/8-in. big-bore wells is designed to handle hydrogen sulfide (H2S) and carbon dioxide (CO2) corrosion, hydrochloric acid (HCl) stimulation, and environmental cracking. The wells must accommodate high flow rates and through-tubing intervention. The full-length paper describes the technical development of an optimized operational envelope for L-80 carbon-steel tubulars for these high-rate gas wells in Qatar.

Introduction

The North field is offshore to the northeast of Qatar in the Arabian Gulf. It is the world's largest nonassociated-gas field, with reported reserves of more than 900 Tcf. The sour, abnormally pressured natural gas is contained in the massive Khuff carbonate formation. RasGas currently has 11 offshore platforms in the North field.

Two different types of production wells have been used on these platforms. The first type is a 7-in.-monobore well with 7-in. tubing and liner made of L-80 carbon steel. The second well type is referred to as the 9 5/8-in. optimized big-bore (OBB) well comprising 9 5/8×7 5/8-in. tapered production tubing made of L-80 carbon steel and a 7-in. liner made of Alloy 28 corrosion-resistant alloy (CRA). More than 60 9 5/8-in. OBB wells have been drilled and completed to date. The wells are more than of 9,000 ft deep with deviations from vertical of as much as 70°.

Well deliverability for the 9 5/8-in. OBB wells is greater than for the 7-in. monobore wells, and on the basis of reservoir modeling, it was determined that the target project production could be achieved with 25% fewer wells. At equivalent production rates, the 9 5/8-in. OBB wells have less wellbore pressure loss, resulting in an extension of the plateau life without surface compression.

All of these wells must be designed to handle H2S and CO2 corrosion, HCl stimulation, and environmental cracking. All of the acid used to stimulate these wells contains corrosion inhibitors to mitigate corrosion in the tubing and liners during acid stimulation. The tubular materials are designed to withstand the corrosive production environment on their own without chemical inhibition. Early in the design phase of the 9 5/8-in. OBB wells, the use of CRAs was considered for both the tubing and the 7 in. liner. However, it was determined that CRA materials could not be fabricated (on a commercial scale) to 9 5/8-in. diameter in usable lengths because of tubular-mill manufacturing limitations, so carbon steel would have to be used to enable the 9 5/8-in. OBB production advantages.