Study of Factors Causing Annular Gas Flow Following Primary Cementing

Abstract

A model of a cemented annulus was used to evaluate the pressure transmitted and maintained by a static column of cement after placement and until set conditions were reached. Pressure is lost during cement hydration, resulting in annular gas flow. Laboratory and field tests were conducted on a new compressible cement capable of maintaining more stable annular pressure against the formation. Introduction Annular gas flow has been a problem in the industry for many years. One of the first recognized annular gas flow problems occurred during cementing of gas storage wells in the mid-1960's. The severity of this problem has varied from a nuisance factor to one that can be very significant. This paper reviews various types of annular gas flow in different areas of the country and considers the cost currently being spent in remedial cementing operations.Research on the problem of annular gas flow is not new. A, brief summary of previous research and field applications are included.Although much effort has been put forth on annular gas flow problems, until recently no new solutions with positive results have been found. One reason for this has been some lack of understanding of the phase behavior of cement slurries. A 4-year research program has led to the development of a new compressible cement system. Results of its application in several field areas are reviewed. Field History on Annular Gas Flow Various types of annular gas flow problems occur in many different producing areas. Following are a few of the problems encountered in different areas, which show its general nature. Houston Area - Offshore (High Island) Formations in the High Island area are Pleistocene sequences. In this area, annular gas now has been encountered on surface and conductor strings. The annular gas flow can result from shallow gas sands or sands that have been pressurized because of similar flow from higher-pressured zones at greater depths. Annular gas flow in some areas of this field are more pronounced on intermediate production strings and liners. The problem is not limited to deviated holes but, in some cases, may be aggravated by hole angle. In this area, it is common to have gas flow back to surface within 0.5 to 1.5 hours after the plug is bumped. This occurs on pipe strings that are cemented back to surface as well as those where cement is brought back only into the interannulus of the last cemented string.Uncontrolled gas flow from shallow zones or deeper high-pressure and high-deliverability zones outside the cemented surface pipe can cause gas blowout at some distance from the cemented annulus on the ocean floor. Often it has been necessary to bring primary cement back short of the surface and then conduct an annulus squeeze job on the cement top in an attempt to shut off gas flow to the floor of the platform. This, however, does not always prevent the loss of gas into shallow sands from the lower zones. A survey on remedial cementing operations in this area for the past several years indicates that cost has varied from $20,000 to $350,000/well. JPT P. 1427＾