Transient Pressure Unloading - A Model of Hydrostatic Pressure Loss in Wells after Cement Placement

Abstract

Abstract Hydrostatic pressure loss (unloading) after cement placement is widely considered a reason for early gas migration in wells. In the presence of high-pressure zone, the unloading leads to pressure underbalance causing gas invasion and flow in the cemented annulus. The work, reported here, is an attempt to analyze and model pressure unloading as a propagation of pressure transient resulting from downhole fluid loss - a primary mechanism of slurry volume reduction. (Experiments, discussed here, had shown that another mechanism of volume reduction - volumetric shrinkage is not at work early enough to contribute to the pressure unloading process.) The pressure-unloading model has been validated using data from field experiments in the instrumented wells. The theoretical results matched experimental data showing exponential trend of the downhole pressure loss. Also, the model was used in a sensitivity study to identify parameters controlling the process. The study revealed that large annuli with high water loss would give rapid and large reduction of hydrostatic pressure in the cement column and more likely intrusion of gas. The observation implies that surface holes should be more sensitive to gas migration than deep holes and the cement slurry filtration should be minimized in surface holes. Another finding showed that compressible annular cement systems would be more tolerant to fluid loss in terms of losing hydrostatic pressure than the low-compressibility systems. The observation emphasizes the importance of annular system compressibility control. Introduction Several accidents in wells on the Outer Continental Shelf (OCS) have occurred shortly after cementing while rig personnel waited for cement to set. Data from these areas have been accumulated by the US Mineral Management Service (MMS) during the last twenty years. In all these events, the diverter system was not completely nippled down before the cement developed sufficient compressive strength. Typically, a few hours after cement placement, a well would start flowing gas. Flow of this type is difficult to control and - if not eliminated - may cause loss of well integrity, spillage, or blow-out. The problem is known as "early gas migration" or "shallow gas flow." The data has been recently analyzed and theoretically evaluated.(1) In view of limited available data, analysis of field cases of flow after cementing has been limited to qualitative circumstantial observations regarding common features of the affected wells such as: casing strings are typically set at shallow depths; hole sizes are relatively large; and; cement slurries are displaced to the surface. Neither of these features, however, could be considered uniquely associated with gas flow which has been shown by comparing "non-flowing" and "flowing" wells. The average flowing time was 4.6 hours after cement placement. Several wells exhibited the problem at the same time that the displacement of the cement slurry was completed. Depths of these wells averaged 3,412 ft; a well over 5,000 ft was a rare case. The casing sizes (surface casings or intermediate casings) ranged from 10.75 in. to 20 in. A typical sequence of events leading to flow after cementing can be summarized in the following steps:Cement is pumped and displaced successfully; the job appears to proceed without problems.After a few hours of WOC, the diverter/BOP stack is nippled down and the well starts to flow.The diverter is nippled up again; the well is diverted in an attempt to control the flow.Even if the diverter does not fail, the well is flowing and is becoming more difficult to control.Various means of restoring control over the well are attempted, e.g., circulating heavy mud through tubing into the annulus, diverting the well, closing the well.In case of severe flow, the rig is evacuated.