A Poroelastic Analysis to Address the Impact of Depletion Rate on Wellbore Stability in Openhole Horizontal Completions

Abstract

This paper highlights the development of a coupled poroelastic geomechanical and fluid flow model which incorporates field and lab data with the objective to constrain the full in-situ stress tensor and rock strength in order to predict the stability of open hole horizontal completions during reservoir depletion. Results of a four-year comprehensive testing and monitoring program conducted to assess the extent of hole instability during shut-in and flowing periods1 indicated that there was no immediate hole collapse. However, the study revealed the need to assess the long-term impact of reservoir depletion and pressure drawdown on wellbore stability. The results of this study indicate that the in situ stress state can be characterized by a normal faulting environment with low differential stresses in which the maximum principal stress is approximately equal to the overburden. Furthermore, a detailed analysis of wellbore stability during production supports openhole completion for horizontal wells under the condition that reservoir depletion is limited to a maximum pressure drop of 1,500 psi. This finding is independent of well azimuth. Pressure drops exceeding 1,500 psi in the reservoir are likely to cause considerable wellbore instabilities. These results were achieved under the assumption of moderate to more pronounced amounts of drawdown (500–1000 psi) in the near wellbore region. The study also highlighted that laboratory-derived rock strength values from triaxial tests, are low and are not consistent with the drilling and production experiences to date in the field. Rather, the formation appears to behave in a plastic manner that strengthens the wellbore. Introduction Hole stability concerns in the Shu'aiba reservoir, Shaybah Field, Saudi Arabia, first surfaced during the drilling and logging of two development vertical evaluation/production wells where a logging tool was stuck in one well due to tight hole, and indications of tight hole were encountered while drilling another well. The two incidents signaled the need to investigate hole stability in the Shu'aiba reservoir. A review of all the vertical delineation wells drilled in the 1960's for problems associated with hole fill and/or collapse during drilling and production found no conclusive evidence for stability problems. Cores taken in the mud-rich, high porosity rock of the Shu'aiba formation have been described as having a "toothpaste-like" texture and behavior. Preliminary laboratory rock mechanics studies indicated that the Shu'aiba carbonates are mechanically weak with the majority of the rocks tested yielding very low strength values (less than 2000 psi) when compared to samples from other carbonate reservoirs. In light of the gathered field and geomechanics data, a comprehensive hole stability monitoring program was formulated and initiated with the objective to investigate the extent and implications of hole stability on field development and deliverability. Results of which are summarized in SPE 565081 indicating, at least in the short term, no impact of production on wellbore stability. Furthermore, a long term study, which is the focus of this paper, was initiated to constrain the full in situ stress tensor (i.e., orientation and magnitude), reservoir pore pressure, and rock strength in order to build a geomechanical model of the Shaybah Field and predict the stability of openhole horizontal wells during reservoir depletion. To achieve these goals, a broad suite of available data from wells drilled in Shaybah Field were utilized such as electrical FMI image data, four-arm caliper logs, minifracs, wireline logs (e.g., density, neutron, and sonic logs), and pore pressure information obtained by direct measurement were studied. This data - sufficient to constrain the full stress tensor and pore pressure - was then augmented by information on uniaxial compressive strength derived from laboratory measurements and log data to build the full geomechanical model, which was then used as a basis to predict wellbore stability during reservoir depletion.

Pore pressure stress coupling in 3D and consequences for reservoir stress states and fault reactivation

“…The effects of PSC on stress magnitudes and tectonic stress regimes should be taken into account in planning additional wells for secondary or tertiary production measures. Furthermore, the PSC methodology should be considered for the early times of injection when (Teufel et al, 1991), Groningen (Hettema et al, 1998), Shaybah (Salamy and Finkbeiner, 2002), Gunung Kembang (Dwi Hudya et al, 2007), Eugene Island Block 330 (Finkbeiner and Zoback, 1998).…”

Use of X-Ray CT to Measure Pore Volume Compressibility of Shaybah Carbonates

“…Tests also indicated that there was a general decrease in strength with increased porosity and that most of the core samples were behaving in a ductile manner. Salamy and Finkbeiner (2002) remarked that the reason all the horizontal wells remained open was because of the "yield zone" behavior of borehole instability, which keeps the rock in the immediate vicinity of the borehole in a failed but semistable state. This instability has the potential to cause borehole restriction, elongation, fill, washout, and, eventually, borehole collapse .…”

“…Extensive petrophysical analysis has been conducted on Shaybah rock samples (e.g., Okasha et al 2000;Okasha and Funk 2002;Okasha et al 2003;Okasha et al 2005;Okasha et al 2006;Hughes et al 2004;Salamy et al 1999;Salamy and Finkbeiner 2002). These include conventional measurements (porosity, permeability, grain density, and CT-scan-based bulk density/porosity), special-core-analysis measurements [relative permeability, wettability, mercury capillary pressure, electrical properties, and nuclear magnetic resonance (NMR)], and geologic core descriptions [XRD, XRF, fluorescence, thin-section petrography, scanning electron microscopy (SEM), and visual core descriptions].…”

“…The presence of a large overlying gas cap and a relatively weak aquifer dictated the use of horizontal completion in Shaybah to minimize the potential of early gas breakthrough while achieving economically desirable production rates. During drilling of these horizontal wells, there were two reported cases of tight-hole situations, which led to extensive borehole-stability monitoring being implemented for several years (Salamy et al 1999;Salamy and Finkbeiner 2002).…”

“…Preliminary laboratory rock-mechanics studies indicated that the Shu'aiba carbonates are mechanically weak, with the majority of the rocks tested yielding very low strength values (less than 3,000 psi) when compared to samples from other carbonate reservoirs (Salamy and Finkbeiner 2002). On the basis of laboratory-derived rock-strength data from triaxial tests, the formation appears to behave in a plastic manner that strengthens the wellbore.…”