Ever increasing pressure from environmental groups government agencies hasforced many operating and service companies to take a serious look at surfacecasing vent flows and gas migration. Concerns regarding well abandonment andlease reclamation, as well as aquifer contamination and green house gasemissions, have increased attention to the problem.
This problem has plagued the oil and gas industry for many years. Experimentsusing various techniques to remedy it have been costly and have consistentlyhad low rates of success. Wireline logging high density perforating andcement/resin squeezing have been applied. The gas zones usually have very lowpermeability and swelling clays, preventing feedrates with water. Consequently, cement squeezing is virtually impossible due to large cement particles bridgingand hydrating immediately at the perforation; often no cement is squeezed intothe formation. Furthermore, shallow formations may be fragile and can fractureunder a hydrostatic cement column. Multiple gas sources, source locations andmisleading log interpretation have also contributed to poor successrates.
A new technique s described here, which is cost effective and meets regulatoryand environmental requirements. It consists of formation evaluation and theapplication of abrasive hydro-jetting hydraulic fracturing, and fine particlecement squeezing. Field results in Southern and Central Alberta show that thetechnique is highly successful; surface casing vent flows and gas migrationhave been terminated.
Whenever a hole is drilled, there is the possibility that fluids previouslytrapped by impermeable layers will migrate to shallower zones or to surface.Unsuccessful primary cementing may leave voids and channels in the cementsheath, allowing fluids to migrate. As a result many wells leak gas to surfacethrough soil gas migration and/or surface casing vent flows (Fig. 1).
In many areas of Alberta and Saskatchewan the first 200 to 400 m of formationconsists of gravel beds, silts, undeveloped shales with swelling clays, and"non-commercial" shallow gas bearing zones. These features make it difficult tomaintain 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 andcement, and through a permeable formation (Fig. 2).
A surface casing vent flow is any measurable flow of gas, water, or hydrocarbonliquids with or without pressure build-up1. It is estimated that oneout of 20 wells in Alberta has pressure on the surface casing vent. Gasmigration refers to gas that migrates to surface through the soil outside thesurface casing around the wellbore. Husky Oil defines a leaker as any wellwith a detectable leak, including those with rates too small to measure withstandard instrumentation. A successful remedial operation is one that stops allleakage for a year or more through at least one freeze-thawcycle(2).
Elimination of gas migration and surface casing vent flows has presented a considerable challenge to the oil and gas industry. Inorder to abandon a lease, government reclamation requires that the site bereturned to its original state. Under current regulations, any detectable gasleakage must be terminated prior to surface abandonment.