A Practical Method for Evaluating Effects of Fracture Charging and/or Ballooning When Drilling High Pressure, High Temperature (HPHT) Wells

Abstract

Abstract Fluctuations in the returned mud volume have often been observed when drilling HPHT wells. There are several contributing factors for this, out of which increased wellbore volume due to elastic deformation and mud taken by natural fractures are considered in this paper. These effects are significant in HPHT wells as a result of the high mud pressure required to control the well. This, however, can give the driller the misimpression that the extra mud volume is lost to the formation due to wellbore breakout and/or fractures. When the mud weight is reduced to prevent such a suspected mud loss, the formation quickly regains its original volume and the "lost" mud is returned. This might again be misunderstood as a kick and the mud weight is increased immediately to prevent the suspected kick. The repetition of this process a few times might eventually lead to an actual wellbore failure. This paper presents a method to estimate the volumetric expansion of wellbores as a function of wellbore pressure. The wellbore near-breakout/fracture pressure, which is of interest for this analysis, is established by considering different failure modes including helical shear, elongated shear and tensile fracture. The increases in wellbore volume are estimated at this pressure as a limit below which the driller should not be fooled by the suspected breakout/kick situation and thus avoid it leading to wellbore failure. The method to estimate the volumetric expansion is based on analytical and numerical approaches. Analyses show that the diametric expansion of the wellbore may be in the range of centimetres at a critical pressure, and thus a deep well may consume a significant number of extra mud barrels before actual breakout occurs. This might be alarming enough to lead the driller to suspect breakout/fracturing in the absence of any analytical guidance. Thus, the paper has presented a novel approach to analyse such a suspected situation during well drilling at HPHT conditions, and the information presented will assist engineers to avoid confusion and manage the well efficiently in such a situation. Introduction Volumetric changes, both positive and negative, in the mud system during drilling operation is commonly termed ballooning. The change in volume, or ballooning volume, can be quite large depending on the well in question, and might as such give a false impression on the surface that the well is either taking a kick or that there is a lost circulation scenario. To address this problem, especially in High Pressure High Temperature (HPHT) wells where the safety margins are often quite small, various studies have been conducted in the past to be able to quantify the ballooning volume. It is commonly accepted that the potential causes for ballooning is mud expansion or contraction due to both temperature and pressure variations, deformation of borehole and casing and loss of mud to natural fractures. Bj rkevoll et al1 and Aadnøy2, based on the same two example wells, conducted a study into the effects of both mud expansion and contraction and the deformation of both borehole and casing by both numerical and analytical means. From these studies it was concluded that the mud ballooning was by far the most significant effect, with only minor contribution coming from the deformation of borehole and casing. Further works on the subject of mud ballooning were carried out by Kårstad and Aadnøy3–9. However, the method used in these works to calculate the effects of elastic deformation is based on the change in well pressure, without any consideration of in-situ stresses. In addition they were considering fairly hard formation types, with E moduli of 30 GPa and 10 GPa. Combining this with open hole radii of 12.25" and 8.5" respectively, the total deformed volume is small, whereas, using the same input data, the method proposed in this paper estimates a higher deformed volume.