Surface Casing Vent Flow And Gas Migration Remedial Elimination-New Technique Proves Economic And Highly Successful
Authors: Saponja J. 
Published: March 2010

Ever increasing pressure from environmental groups and government agencies has forced many operating and service companies to take a serious look at surface casing vent flows and gas migration. Concerns regarding well abandonment and lease reclamation, as well as aquifer contamination and green house gas emissions, have increased attention to the problem.

This problem has plagued the oil and gas industry for many years. Experiments using various techniques to remedy it have been costly and have consistently had low rates of success. Wireline logging, high density perforating and cement/resin squeezing have been applied. The relevant gas zones usually have very low permeability and swelling clays, preventing feedrates with water. Consequently, cement squeezing is virtually impossible due to large cement particles bridging and hydrating immediately at the perforations; often no cement is squeezed into the formation. Furthermore, shallow formations may be fragile and can fracture under a hydrostatic cement column. Multiple gas sources, source locations, and misleading log interpretation have also contributed to poor success rates.

A new technique is described here which is cost effective and meets regulatory and environmental requirements. It consists of formation evaluation and the application of abrasive hydro-jetting, hydraulic fracturing, and fine particle cement squeezing. Field results in southern and central Alberta show that the technique is highly successful; surface casing vent flows and gas migration have been terminated.

Introduction

Whenever a hole is drilled, there is the possibility that fluids previously trapped by impermeable layers will migrate to shallower zones or to surface. Unsuccessful primary cementing may leave voids and channels in the cement sheath, allowing fluids to migrate. As a result many wells leak gas to surface through soil gas migration and/or surface casing vent flows (Figure 1).

In many areas of Alberta and Saskatchewan, the first 200 to 400 m of formation consists of gravel beds, silts, undeveloped shales with swelling clays, and "non-commercial" shallow gas bearing zones. These features make it difficult to maintain borehole stability and zonal hydraulic isolation.

Gas can migrate through four possible passages: between the casing and cement (microannulus), through channels in the cement, between the borehole wall and cement, and through a permeable formation (Figure 2).

A surface casing vent flow is any measurable flow of gas, water, or hydrocarbon liquids with or without pressure build-up(1). It is estimated that one out of 20 wells in Alberta has pressure on the surface casing vent. Gas migration refers to gas that migrates to surface through the soil outside the surface casing, around the wellbore. Husky Oil defines a "leaker" as any well with a detectable leak, including those with rates too small to measure with standard instrumentation. A successful remedial operation is one that stops all leakage for a year or more through at least one freeze-thaw cycle(2).

Elimination of gas migration and surface casing vent flows has presented a considerable challenge to the oil and gas industry. In order to abandon a lease, government reclamation requires that the site be returned to its original state.