Wellbores leaking small amounts of natural gas to the surface are present over some areas of Western Canada. Various conventional techniques have provided limited success in attempting to resolve the problem. This paper summarizes the geological selling, previous Husky efforts and planned future activity in the Lloydminster area.
To try to identify the gas leakage source, a geological review was undertaken. Most Lloydminster area production is derived from the Mannville Group and Therefore, overlying Cretaceous marine sequences have been largely ignored. Of these sediments, portions of the Colorado and Montana Groups contain organic-rich source rocks which have generated biogenic methane in the White Speckled Shales and Fish Scales Zone, and nonconventional to conventional gas reservoirs in the Belly River, While Speckled Shales, Viking, St. Walburg and Spinney Hill Formations. Much of the shale in the up-hole section is poorly consolidated, panicularly in The Lea Park Formation, and contains abundant illite and water-sensitive smectite clays.
Husky has conducted an extensive field testing program over The last 10 years. Results to dare have not been encouraging. Comparing fi1:ld results to lab data indicates The inherent problem is obtaining an effective cement bond to unconsolidated mudstones. Set cement permeability and casing CO cement micro annulii are also explored as leakage paths.
The current thrust of Husky's research is two-fold, to assess the feasibility of a Technical Solution co eliminate gas migration in existing and new wells and to provide a Business Solution that would define acceptable levels of gas migration.
The leakage of natural gas is a widespread problem and affects a large number of wells, particularly in the Lloydminster area where a high well density coincides with extensive agriculture. The gas rates found tend to be low and the most significant impact is productivity damage to a few tens of m2 of land in some wells. Starting in 1991, Husky, Novacor Research, Dowell and Schlumberger embarked on a joint research and field testing program. Schmitz et al 1 describe field testing methods and provide preliminary data that show most wells leak at the rates of less than 1m3/d. This paper will compare leaking wellbore geometries, geology and engineering data to our current field experiences and will briefly outline planned future activity.
Husky applies stringent definitions; a leaker is any well with a detectable leak, including those with rates too small to measure with standard instrumentation, and a successful remedial job is one that stops all leakage for a year or more through at least one freeze-thaw cycle. A short term reduction in gas flow that does not permanently stop the leak is not considered a success.
Most hydrocarbon production in the Lloydminster area originates from Lower Cretaaccous sandstones of the Mannville group. A large number of leaking well sin Lloydminster do not have conventional gas zones detected by neutron-density crossover. Cement squeezes or other remedial work carried out in Mannville zones will not have any effect on gas migration sequences are shaley, have limited commercial potential and have been largely ignored.