Abstract

Gas-storage wells experience gas deliverability decline over the lifetime of the well for a variety of reasons, but most often decline is caused by near-wellbore skin damage. Recent developments in diagnostic techniques enable operators to identify near-wellbore damage and apply a tailored treatment to reverse the deliverability decline. In a Sayre, Oklahoma gas-storage field, gas deliverability was improved 32.5% in 14 wells, using pretreatment damage diagnostic analysis to remove near-wellbore damage with new coiled-tubing-conveyed fluid-oscillation treatments.

A fluid-oscillating tool (FOT) is key to the damage-removal technology. The FOT sends out alternating bursts of fluid to create pulsating pressure waves within the wellbore and formation fluids. These pressure waves help break up near-wellbore damage and restore effective permeability by carrying the fluid past the wellbore into the formation. These oscillating pressure waves are not affected by standoff, which is common with conventional jetting or velocity tools. Kinetic energy in the pressure pulse travels through the wellbore fluid with no appreciable energy loss. The pressure waves expand spherically, providing 360° coverage as the tool is moved through the interval. As damage is removed, the waves penetrate deeper into the formation.

This paper presents a technical description of the processes incorporated to remediate damage in the Sayre field and the technology used in the oscillation treatments. A case study from West Virginia is also presented. Both cases illustrate pretreatment planning, job design, application procedures, and results.

Introduction

Each year, a wide variety of damage mechanisms cause gas-storage operators to experience approximately 5% deliverability loss.1 Recently, a joint industry study identified many of these damage mechanisms, and a diagnostic technique and damage-removal process has been described in detail.2 The damage-removal method uses coiled tubing (CT), a jetting nozzle, and a tailored treatment fluid to remove the damage that limits gas-well deliverability.

Hundreds of coiled-tubing cleanouts (CTCO) have been performed on gas-storage wells throughout the United States using the diagnostic process to identify damage mechanisms and assist in the design of appropriate procedures and treatment fluids.3 In most of these treatments, either a high-pressure blasting tool (HPBT) or a wash nozzle was used to clean the wellbore. In addition to cleaning the wellbore, various methods have been incorporated to pump additional treatment fluids into the formation to remove skin damage.

The FOT, used on over 3,000 wells throughout the world, had never been used in conjunction with the damage-diagnostic procedures for gas-storage wells described in this paper.

Previous tests performed with the FOT indicate that energy from the tool cleans the exposed formation surface (as blast-and-wash nozzles do) and more effectively penetrates into the formation to remove near-wellbore skin damage. Blast-and-wash nozzles alone have not proven effective in penetrating into the formation to remove skin damage. It is well-known that many of the damage mechanisms in gas-storage wells are not just at the formation or wellbore surface, but are embedded back into the formation itself in the near-wellbore area.4

To determine whether gas deliverability could be improved over historical CTCOs, it was proposed to use the FOT with a blasting tool and the in-place, damage-diagnostic process. Previous CTCOs used the HPBT alone or the HPBP in conjunction with selective injection packer (SIP) systems to convey customized treatment fluids into the formation to remove skin damage. In this project, past methods would be compared with the oscillation tool used alone.

FOT Theory

The FOT (Fig. 1) is used in conjunction with CT and is based on patented fluid-oscillator technology. Fluid oscillation produces emissions of alternating bursts of fluid that create pulsating pressure waves within the wellbore and formation fluids. These pressure waves can break up many types of near-wellbore damage, helping restore and enhance the permeability of the perforations and near-wellbore area. The pressure waves expand spherically, providing 360° coverage while the tool is moved through the interval. As damage is removed, the waves penetrate deeper into the formation (Fig. 2).

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