Damage Mechanisms In Gas-Storage Wells
Authors: Yeager V.J. , Blauch M.E. , Behenna F.R. , Foh S.E. 
Published: December 2007

Abstract
This paper presents the results of a 2-year, GRI/DOE-funded project undertaken to determine the damage mechanisms that affect the 15,000+ U.S. gas-storage wells. The paper identifies damage mechanisms based on the evaluation and field testing of 32 wells from 12 reservoirs. The 12 reservoirs were selected as a representative cross section based on a statistical analysis of existing storage database information.
Downhole diagnostic field results presented include (1) well test analysis to determine quantitatively if damage existed, (2) downhole video to observe the wellbore and formation areas, (3) physical sampling of downhole liquids and solids, and (4) rotary sidewall core samples of the wellbore face as a "biopsy" of the storage formation.
This paper incorporates (1) guidelines for the candidate well selection processes and various strategies/test procedures that may be used to determine damage based on the reservoir and available information, (2) a systematic approach for field and laboratory testing to identify causes of deliverability reduction, and (3) general conclusions for the overall storage population based on the representative candidates.

Introduction
In the more than 400 U.S. storage reservoirs that represent approximately 15,000 individual wells, most gas-storage operators experience an average loss in deliverability of approximately 5% in 1 year. This decline rate, based on reported American Gas Association (A.G.A.) deliverability capacity, translates to approximately 3 Bcf/D each year for the entire industry. The Mauer Engineering study estimates the spending to recover or replace deliverability and maintain the current rate of deliverability at tens of millions of dollars per year. These expenditures include both drilling and stimulation/remediation. However, drilling is the more costly method of retaining or recovering this loss, and as the demand for storage increases, the need to improve and maintain deliverability in existing wells also increases. Two problems exist regarding the cost of recovery and replacement of deliverability. First, a significant portion of this money is expended without a clear understanding of the damage being addressed. Second, while operators may understand the various mechanisms of damage that could account for the loss, they have no diagnostic approach available that will help them determine which mechanism is responsible for the loss in a specific circumstance. Therefore, the choice and design of remedial and preventive measures is less effective than it could be if a better understanding of the problem existed.
This research effort is designed to produce an understanding of the formation damage mechanisms responsible for deliverability loss and to examine the effect these mechanisms have on deliverability loss in a broad spectrum of gas-storage fields and reservoirs. The project objective is to provide (1) definitions of the mechanisms responsible for loss of deliverability in storage wells, (2) an outline of testing procedures that operators can use to deduce the type of damage mechanism, and (3) the basis for identifying procedures to prevent or remove damage.

Candidate Selection
The candidate selection process (Fig. 1) involves integrating information and expertise designed to select candidates based on reservoir type characterization, deliverability loss assessment, and well history/well records review. The objective of this task is to use existing data and classification of reservoir types outlined in the Mauer Engineering study (reservoir types R1 through R12) and document the magnitude of deliverability losses within fields operated by the cooperating companies.
From this information, the broadest contrast examined was sandstone vs. carbonate rock type. In general, gas-storage reservoirs in sandstones make up 69% of the population. Carbonates comprise 27% and other storage media such as salt caverns and coal make up 4%. This data is illustrated in Fig. 2. P. 193^