Analysis of the deep drilling technology in unstable formations at the Semyrenky gas condensate field

The paper presents a general overview of deep drilling in unstable formations at the Semyrenky gas condensate field of the Dnipro-Donetsk Trough, including well design, bottom hole assemblies (BHA), drilling conditions, and drilling muds. Problems encountered during drilling for production casing of Wells 72- and 75-Semyrenky using high-speed drilling methods are analyzed. The relationships between the rate of penetration and disturbed rock stability, volume excess and depth, as well as consistent empirical patterns in changes in mud properties and depth are established. With these technical and economic performance indicators for well drilling are given, elements of a borehole stability management strategy were defined, the principles of mud selection for drilling through problem zones are validated. The paper discusses the requirements to a mud hydraulics program to reduce the erosion of borehole walls, specific borehole preparation techniques, such as reaming and gauging, for drilling in problem zones, and alternative options to ensure borehole stability. Keywords: borehole stability; statistical models; hole gauging; hole geometry; drilling mud; BHA.

Delivered Successfully an Open Hole Side-Track from Very Challenging Formation Through Very Narrow MD Window: A Case Study from Kuwait

This article presents a unique case study where operating company, Kuwait Oil Company (KOC), decided to make an attempt to perform open hole side-track through a very narrow side-track window along with other exiting conditions such as severe downhole losses and drill through very challenging formation. To deliver such project in first attempt requires very detailed planning, close coordination with various service partners such as directional drilling and cementing. Placing a good side-track cement plug in such formation was a challenge, and 2nd challenge to get kicked off from this narrow window in first attempt which was the key. In case of failure, whip stock option has to be planned as a contingency, which possess new challenging of opening a depleted zone leading to commingling low/high pressure formation which could cause a complicated problem such as borehole stability, leads to stuck pipe problem. Failure to side-track from open hole could end of planning to drill extra hole which required extra casing string to run which will put this project well over AFE and heavily impact on well objective. This open hole Side-track was planned because while drilling original hole (12 ¼" hole section) close to planned well TD, experienced complete losses. In attempt to cure the losses, LCM was pumped with no success. Performed thixotropic cement plug job for losses control. While performing thixotropic cement plug job, the cement flash set before finishing the displacement, leading to stuck string. After backing off string and fishing attempts, unable to recover the fish completely. Fish left in hole leading to only 68 ft of open hole window available to side-track where performing a cement job was impossible due to severe losses. Only way to secure the well is to try for open hole side-track. With existing sever loss situation for initiating open hole side-track was a serious challenge due to lack of side force and flow restriction to initiate the side-track. Extensive pre-job planning, peer review and risk assessment was done in coordination with various service partners to deliver such challenging side-track. A hazard analysis decision tree was established to pinpoint the risks and appropriate mitigation measures along with contingency plan put in place. A detailed side-track guidelines was shared and review with the field crew. The wellbore was successfully side-tracked through a challenging reactive shale formation in a first attempt using a customized kick-off BHA, which not only helped to avoid loss zone in side-tracked hole but also provide additional cost savings to the company. The good hole condition at the side-track point was important to enable smooth passing of the following directional BHA to achieve directional goals.

Integration of Acoustics and Geomechanical Modelling for Subsurface Characterization in Tectonically Active Sedimentary Basins: A Case Study from Northeast India

Drilling wells in the remote northeastern part of India has always been a tremendous challenge owing to the subsurface complexity. This paper highlights the case of an exploratory well drilled in this region primarily targeting the main hydrocarbon bearing formations. The lithology characterized by mainly shale, siltstone and claystone sequences, are known to project high variance in terms of acoustic anisotropy. Additionally some mixed lithological sequences are also noted at particular depths and have been identified at posing potential problems during drilling operations. Several issues became apparent during the course of drilling the well, the main factor being consistently poor borehole condition. An added factor potentially exacerbating the progressively worsening borehole conditions was attributed to the significant tectonic activity in the area. To address and identify these issues and to pave the way for future operations in this region, a Deep Shear Wave Imaging analysis was commissioned to identify near and far wellbore geological features, in addition to addressing the geomechanical response of these formations. In this regard, acoustic based stress profiling and acoustic anisotropy analysis was carried out to estimate borehole stability for the drilled well section and provide insights for future drilling plans. Significant losses were observed while drilling the well, in addition to secondary problems including tight spots and hold ups and consequently the well had to be back reamed multiple times. Of particular note were the losses observed while transitioning between the main formations of interest. The former consisting relatively lower density claystone/siltstone formations and the latter, somewhat shalier interlayered with sandstones, displaying a generally higher density trend. This transition zone proved to be tricky while drilling, as a high density sandstone patch was encountered further impeding the drilling ROP. Overall, both formations were characterized by significantly low rock strength moduli with the exception of the sandstones projecting characteristically higher strengths. In light of these events, analysis of integrated geological, geomechanical and advanced borehole acoustic data analyses were used to identify the nature of the anisotropy, in terms of either stress induced, or caused by the presence of fractures in the vicinity of the borehole. The extensive analysis further identified sub-seismic features impeding drillability in these lithologies. Further, the holistic approach helped characterize the pressure regimes in different formations and in parallel, based on corroboration from available data, constrained stress magnitudes, indicating a transitional faulting regime. Variances in stress settings corresponded to the depths just above the transition zone, where significant variations were observed in shear wave azimuthal trends thereby indicating the presence of potential fracture clusters, some of which were revealed to be intersecting the borehole thereby causing stress. The analysis shed light on these near well fractures- prone to shear slip, causing mud losses during drilling while drilling with high mud weights. Finally, the encompassing multiple results, an operational mud weight window was devised for the planned casing setting depths. Given the presence of numerous fractures, the upper bound of the operational mud window was constrained further to account for the presence of these fractures. In summary, an integrated approach involving a detailed DSWI study in addition to traditional geomechanics has brought about new perspectives in assessing borehole instability. By actively identifying the sub surface features, (sub seismic faults and fractures) decisions can be taken on mud weight and optimizing drilling parameters dynamically for future field development.

A new method for layout layer optimization of long horizontal borehole for gas extraction in overlying strata: a case study in Guhanshan coalmine, China

The large emission of pressure-relief gas in coal mining will cause atmospheric greenhouse effect. Arranging long horizontal borehole (LHB) to extract the gas is an effective solution. However, the determination of LHB layout layer in strata has a decisive effect on efficient gas extraction. A "zone-block" theoretical method for determining the LHB layout layer, in present study, was proposed by combining physical simulation test, theoretical analysis and engineering application. Three processes of the method were presented. Firstly, stable fractured subzone (SFSZ) was the optimum zone of the LHB layout based on the analysis of mining-induced fractures distribution and the borehole stability, and spatial location boundaries of the SFSZ in overlying strata were defined. Secondly, the SFSZ was divided into nine-grid blocks, the LHB layout suitability rate of each block were determined according to borehole stability rate, fracture permeability rate and gas accumulation rate. Finally, the LHB drilling could be conducted sequentially according to the layout suitability rate of each block in SFSZ. Field application results show that: the maximum and average amount of pure gas extracted through single borehole arranged in block I can reach up to 5.52 and 2.43 m3·min-1, respectively; and the pure amount in the entire extraction stage of the borehole is 2.53 and 6.69 times of boreholes arranged in blocks II and III, respectively. The proposed method can effectively determine the LHB layout layer in strata, so as to improve the gas extraction efficiency and ensure safe and green mining.

Characteristic Law of Borehole Deformation Induced by the Temperature Change in the Surrounding Rock of Deep Coalbed Methane Well

The borehole stability of the coalbed methane (CBM) well has always been vital in deep CBM exploration and development. The borehole instability of the deep CBM well is due to many complicated reasons. The change in the surrounding rock temperature is an important and easily overlooked factor among many reasons. In this research, we used methods that include experiment and numerical simulation to study the characteristic law of the borehole deformation induced by the changes in the surrounding rock temperature of deep CBM well. The experimental results of the stress–strain curves of five sets of experiments show that when the experimental temperature rises from 40 °C to 100 °C, the average stress when coal samples are broken gradually decreases from 81.09 MPa to 72.71 MPa. The proportion of plastic deformation in the entire deformation stage gradually increases from 7.8% to 25.7%. Moreover, the characteristics that some key mechanical parameters of coal samples change with the experimental temperature are fitted, and results show that as the experimental temperature rises from 40 °C to 100 °C, the compressive strength, elastic modulus, and main crack length of coal samples show a gradually decreasing trend. By contrast, the Pois-son's ratio and primary fracture angle show a gradually increasing trend. Moreover, the relativity of the linear equations obtained by fitting is all close to 1, which can accurately reflect the corresponding change trend. Numerical simulation results show that a high temperature of the surrounding rock of the deep CBM well results in a high range of stress concentration on the coal seam borehole and high deformation.