Abstract

Gas migration in cemented annulus of producing wells is considered the mechanism of sustained casinghead pressure (SCP) - an irreducible annular pressure at the casing head. SCP is a hazard to well integrity, potential cause of blowouts and a source of air pollution. From the regulatory standpoint, SCP disqualifies wells for safe operation. In principle, wells with SCP cannot be produced, or plugged and abandoned (P&A) without remedial operations. At present, the SCP removal is either capital intensive, as it requires well's workover with a rig, or less expensive but little reliable with rig-less techniques such as CARS or Bleed-and-Lube. However, gas migration in some wells may cease with little intervention when the migrating gas is wet and flow channels are small.

In this paper, we present our study of a self-plugging technique based on phase behavior of wet gas flowing in the small fissures of leaking cement. A prolonged drop of pressure resulting from opening the casing head to atmospheric pressure may bring the gas in the leaking cement below dew point and set off its condensation. In the process, the condensate liquid would plug the cement fissures and stop the gas flow. In the study, we simulated the plugging process for different well conditions by considering a balance of the capillary, hydrostatic and formation pressures superimposed over the PVT gas envelope.

The results show that surface-induced plugging is possible and could be analytically assessed for a SCP well before deciding to use other expensive methods. Also, shown in the paper is the self-plugging assessment procedure, example of its application, and a discussion of availability of the input data for the procedure.

Introduction

In the USA, the Minerals Management Service (MMS) regulates wells with pressures at casinghead on the Outer Continental Shelf. When wells produce for a certain period of time, small leaks may develop from tubing or casing string due to aged thread connections, corrosion, thermal-stress cracking or poor primary cementing. On surface, the leaks can be detected or tested by the casinghead gauges and flow meters, and some of the leaks may cause the sustained casing pressure (SCP). Wells with SCP are menaces to the worker's safety and environment protection.

A. T. Bourgoyne, Jr. et al.1,2 overviewed the SCP problem and methods for removal. The most direct method involves perforating or cutting the casing to squeeze cement. The method has proven costly and difficult because it requires moving in a workover rig. Other methods such as Casing Annulus Remediation System (CARS) or Bleed-and-Lube, involve injecting high-density fluid into the affected annulus. Somei Nishikawa et al. 3,4,5 evaluated experimentally the injected and annular fluids. The study showed the importance of fluid compatibility testing prior to the injection. Rong Xu et al. 6,7,8 built a mathematical model of a well with SCP and demonstrated various patterns of casing pressure response to the diagnostic bleed-off/build-up tests.

Kevin Soter et al. 9 described modern workover techniques for SCP removal, such as window milling operation and termination of inner casing.

This content is only available via PDF.
You can access this article if you purchase or spend a download.