Abstract

The underground gas storage (UGS) industry uses more than 400 reservoirs and 15,000 wells to store and withdraw natural gas and is thus a significant contributor to gas supply in the United States.1 Previous work indicates that many gas storage wells show a loss of deliverability each year due to numerous damage mechanisms and it is estimated that tens of millions of dollars are spent each year to recover or replace this lost deliverability.2

Prior Gas Technology Institute (GTI) studies3,4 have been aimed at qualitatively reviewing completion and deliverability enhancement techniques used in the storage industry, identifying specific damage mechanisms present in storage reservoirs, and establishing procedures for damage mechanism identification. This paper presents key results of a major new study sponsored by the Gas Technology Institute (GTI) that focused on a comprehensive, quantitative assessment of the effectiveness and longevity of the various stimulation treatments employed in the UGS industry.

Specifically, this paper summarizes the major results of the most comprehensive quantitative study conducted to date of stimulation treatments employed in the UGS industry. The study involved seven operators and 23 reservoirs (14 carbonate and 5 sandstone). Data were collected, entered into the database, and reviewed for 381 stimulations in 365 wells. Deliverability data were entered for 159 stimulations in 155 wells (67 in carbonate reservoirs and 88 in sandstone reservoirs) with sufficient data for inclusion in the study.

The most popular stimulation treatments currently employed in the UGS industry are identified, the effectiveness and longevity of various treatment types are quantified and compared, and a new quantitative indicator designed to capture the results of effectiveness and longevity calculations in a single benchmark indicator is presented. In addition, the frequency and duration of cleanup effects for various treatment types and the post-stimulation decline rates typically observed for various treatments are also quantified and compared.

The implications of conclusions reached in the study, as they relate to future UGS R&D efforts, are also discussed, and specific areas of additional study are recommended.

Introduction

In prior GRI studies, similar issues to those addressed in our study have been examined to a very limited extent. Specifically, in a prior GRI investigation by Mauer et al.,5 Mauer conducted a broad survey of operators to determine deliverability decline rates, the types of deliverability enhancement treatments employed in the UGS industry, and the success of these techniques. All of this work was based solely on the qualitative opinions/estimates of the operators polled. No quantitative, well-specific data were collected to verify these estimates.

One primary objective of the new GTI study is to perform a comprehensive, quantitative assessment of the effectiveness and longevity of the various stimulation treatments employed in the UGS industry. In the process of achieving this objective, we also gained considerable insight into two related areas important to UGS operators: post-stimulation cleanup effects and post-stimulation deliverability decline rates. This study represents the most comprehensive quantitative assessment to date of stimulation treatments employed, cleanup times experienced, and post-stimulation decline rates observed in the UGS industry.

Methodology

To accomplish our objective, we developed a methodology to quantify a storage well's deliverability over time using standard backpressure test data. This methodology requires determination of the well's deliverability indicator, DI (defined as the deliverability potential at a specified delta-pressure squared value, Pr2 - Pwf2, that is representative of actual operating conditions), at least once before stimulation treatments and several times after a stimulation treatment. This was accomplished by using backpressure curves and/or the associated equations supplied by the operators. Typically, operators collect DI information only once or twice a year, and often less frequently.

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