A Method for Evaluation of Risk of Continuous Air Emissions From Sustained Casinghead Pressure

Abstract

This paper was part of a student paper session at the conference.This paper was included in the proceedings as STUDENT5. Abstract Well cement problems such as small cracks or channels can result in gas migration and lead to sustained pressure at casingheads (SCP). Regulations require removal of significant SCP while tolerating small SCP during production operations. However, SCP of any size must be removed prior to well plugging and abandonment (P&A) as it is believed that even very small leaks might lead to continuous emissions of gas to the atmosphere. In some wells, however, small gas leaks may diminish in time as the gas migration channels are plugged off by the gas condensate. Thus, the objective of this work was to develop a method for computing potential of self-plugging in wells with SCP using priciples of phase behavior and theory of two-phase flow in small channels. Introduction In the industrialized world, today, particularly over cities, air pollution is being contonously generated by smoke from automobiles, factories and hundreds of other sources. There are noxious gases like carbon monoxide, sulfur dioxide and oxides of nitrogen. Air pollution does millions of dollars worth of damage to human being and agricultural crops. The United States, with the world's largest economy, is also the world's largest single source of anthropogenic (human-caused) greenhouse gas emissions. Quantitatively, the most important anthropogenic greenhouse gas emission is carbon dioxide, which is released into the atmosphere when fossil fuels (i.e., oil, coal, natural gas) are burned. In addition to carbon dioxide, oilfiled production facilities are a source of natural gas emissions from gas dehydration units, leaking valves, gaslines and oil storage tanks.[1, 2, 3] Also, natural gas leaks may occur from tubing or casing string often due to poor thread connection, corrosion, or thermal-stress cracking[4] The Clean Air Act of 1970 categorized emission sources into two types: mobile source such as automobile, ships, airplanes and stationary emission source which includes refineries, well facilities, oil and gas tanks, etc. Also, with the direction of the act, EPA established National Ambient Air Quality Standards and calculation methods based on air concentration and emission rates to assess the risk of air pollution.[5, 6 7] For the oilfield production facilities, emission rates are computed using published values of emission factors - multipliers specific for the equipment installed in the facility. [8] There are no emission factors, however, for leaking petroleum wells. Considerable number of producing and abandoned wells with SCP constitute a potential new source of continuous natural gas emission from failed casingheads due to poor cementing and external gas migration. The gas migration in cement can be diagnosed with the SCP testing procedures involving casing pressure bleed-off followed with pressure buildup. The tests provide data for assessing gas emission rates - the first component of risk analysis. (The theory ofSCP testing, - developed at LSU - provides estimation of cement leak conductivity and gas source pressure - a two data needed for computing the emission rate.[9]) Another component is the continuity of gas emission, as some wells may cease to emit gas in time due to source depletion or self-plugging with condensate. In condensation, natural gas fractions turn into liquid when pressure and temperature drop below the dew point. The condition of gas condensation could be induced downhole in the cement channel by rapid pressure drop at the surface resulting from failure of containment (casinghead). If the channel is small, condensate liquid will block the gas flow and terminate gas migration. As the gas emissioin from wells may lead to different outcomes. there is a need to develop a method for predicting potential air pollution from the SCP wells. The method should provide prediction of emission rate and - more importantly - the well's potential for self-plugging, estimation of the flowing time and total gas volume emitted to the atmosphere.The method should be theoretically derived by coupling the PVT gas behavior with critical conditions for liquid blockage in the cement's flow channels.