Transient Behavior of Annular Pressure Build-up in HP/HT Wells

Abstract

Abstract Pressure build-up due to fluid thermal expansion in sealed annuli of HP/HT wells can have serious consequences such as casing failure or tubing collapse. To determine whether mitigation was required for a HP/HT development, annular pressures in an appraisal well were studied with a dedicated field test, which consisted of running a pressure/temperature memory gauge in a casing/casing annulus of a well and testing the well several times during a 3-month period, after which the gauge was retrieved and the data were read out. First of all, comparison of the magnitude of the observed annular pressures with the burst and collapse ratings of the casings, shows that annular pressure build-up is a serious consideration in casing design. Such design is to be based on theoretical models for annular pressure build-up. The data acquired with the test serve to validate these models. The data demonstrated that in the lower temperature range (20 to 40°C), on average, pressure development in the annulus agreed reasonably well with theoretical model predictions, based on thermal expansion of the annular fluids and casings, and ballooning and compression of the casing strings. The influence of these factors could be established by analyzing the transient pressure response of the annulus. At higher temperatures the theoretical models overestimate pressure build-up. This is probably to be attributed to the properties of the completion fluids differing from the properties of the base fluid, water. Estimates on the basis of pure water properties can be considered a worst-case estimate for pressure build-up. Leak-off of the annular fluids, which was seen to dominate pressure development during a previous test in a well with a cement shortfall between casings, did not play a significant role in this fully cemented and sealed annulus. Introduction Pressure build-up in tubing-casing or casing-casing annuli is in general undesirable. Although casing design should take into account high pressures at casing head due to e.g. leakage or thermal expansion of the annular fluids, high pressure differences always holds the risk of casing burst or collapse at weak points, leading to loss of production1 or in the worst case, loss of the well2. For this reason most operating companies adhere to annular pressure management schemes for onshore and platform wells, which prescribe bleeding off pressure through the wellhead once a pre-determined pressure level (e.g. 20 percent of the minimum internal yield pressure of the affected casing) is reached. Obviously such schemes cannot be applied to subsea wells since these wells will usually not be equipped with the option to bleed off the annular pressure at distance and to divert the released liquids to a controlled volume. Unmanned platforms and wells at inaccessible locations will face similar problems. Furthermore, in HP/HT wells the high temperatures developing in the well during prolonged production periods aggravate the problem of pressure build-up in the annuli, since the thermal expansion of liquids tends to increase at higher temperatures. Hence design of the casing strings will have to consider these pressures. For that purpose accurate prediction of the annular pressure build-up in casing-casing and casing-tubing annuli is important. Theoretical models to predict thermal annular pressure build-up in annuli have been proposed by a number of authors3,4,5,6. Usually these models take into account thermal expansion of the fluids, "ballooning" and compression of the casings, leak-off of annular fluids and influx of formation fluids. These models will be discussed briefly in the following section. In a first attempt to validate the theoretical models, a field test was set up to study pressure development in the casing-casing annulus of an offshore platform well7. In this particular well however, the inner (production) casing was not cemented into the shoe of the outer casing. The test demonstrated that in this case annular fluid leak-off completely dominated the pressure build-up. Although this indicated that leaving a cement shortfall between casings could be an effective (though not universally applicable) method to combat annular pressure build-up8, the result did not allow any conclusion to be drawn on the overall validity of the theoretical models for pressure build-up. Hence a second test in the sealed annulus of an offshore HP/HT gas well was planned and executed. Set-up and execution of this test will be discussed.