No abstract

New zones in mature fields continue to be actively developed as operators strive to maintain depleting reserves. Many of the world's new reserves are discoveries made below these existing, mature reservoirs. Drilling activities in or near producing or previously abandoned reservoirs often encounter large variations in pressure gradient as depleted layers or low pressured zones are exposed during the drilling process. Zones with pressures inconsistent with the overburden are often encountered, if conventional drilling techniques are used, then the higher mud weight used to hold back the target interval may result in massive losses (lost circulation), differential sticking, sloughing, or collapsing formations in the lower-pressure zone. There is a clear advantage to drilling if we could drill by reducing the drilling window between pore pressure and fracture gradient, using cage stress (particle bridging) or wellbore strengthen (WS) to drill at higher mud weights without losing fluid. This work presents the approach of WS that can be accomplished with treatment of the whole drilling fluid with Lost Preventive material (LPM) that are designed to seal and effectively increase fracture resistance allowing the operator to drill through a weak formation zone successfully with minimal to zero losses. Other models and materials were also investigated to determine their suitability and operational design parameters. Models for LPM application procedure (minimum volumetric requirements, quantitative requirements for filter cake thickness and optimise hydraulic parameters for effective cake deposition) were developed. The advantage of this process is reduction in drilling difficulties and non productive time for operators. Some experimental work was also carried out to determine the port throat of the formation, particle size distribution (PSD), strength & stability of the LPM filter cake, as well as the compatibility of different LPMs with different mud system. Results from case studies were also presented which shows tremendous result in reduction of NPTs via no loss circulation.

Optimization of hole cleaning remains a vital challenge when planning and drilling deviated, high angle and extended reach wells. Hole cleaning depends on a number of factors and as to date most existing models have been deployed in solving hole cleaning problems. However, the flow rate predicted by these models may not be feasible to apply practically in field operations because it gives a pressure exceeding allowable limits of the pop-up valves on the mud pump. This is the major cause of downtime during drilling operations. This research is aimed at adding value to the existing models in achieving better hole cleaning and reduced down time. This was made possible through the use of cutting monitoring model which is a real time and quantitative tool. A case study on a well being drilled in the Niger Delta was conducted whose from which it was observed that within 5800ft to 11500ft, the hole was not properly clean as less cuttings were recovered. This information was used to initiate hole cleaning procedure. From the validation, the results shows Non-Productive Time associated with hole cleaning has a significant drop of 2-5 days when the cutting monitoring model is used in conjunction with the existing models.

The number of multilateral wells drilled in the Niger Delta all geared towards increasing recovery as well as other field development objectives. However, a major challenge facing the successful drilling of these special trajectory wells is hole instability. The cost of instability according to some operators can be as high as $ 600MM per Year. Wellbore instability poses a unique threat in multilateral wells such that the tendency for collapse or fracture increases at the bore hole junctions. Casing deformation, difficulty in through-put of drilling tools and equipments, to mention but a few, are among the major problems associated with this instability. This research revisits the issue of wellbore stability with emphasis on multilateral wells. A modified failure model is developed by including a stress concentration factor based on the junction configurations. The Mogi criterion was applied to account for intermediate stress and predict the minimum mud pressure below which shear failure will occur. A risk and uncertainty factor is attached to this model for sensitive model parameters including stress concentration factors, and lateral bore entry angle. Results from two case studies presented in this work showed that collapse gradient prediction by Mogi based model were more reliable than those of Mohr based model. In case 1, a minimum mud weight of 0.52 psi/ft was predicted to be sufficient to maintain stability. A mud weight of 0.53 was used to drill the lateral section and hole pack offs were encountered at build angle of 30°. However, the Mogi prediction was 0.57 and sufficiently prevented hole collapse. In case 2, the Mogi predictions are all well above the actual mud weight used as against the underestimation with Mohr's.

Borehole instabilities during drilling are more common in shale formations than in most other rock formations. The assessment of in-situ stress and analysis of borehole failure due to instability and weak bedding planes represents one of the most critical factors when evaluating borehole stability that causes borehole failure. Significant amount of research have been done in this area which resulted in various mathematical models about the issue of borehole failure, stability and plane of weakness due to bedding. So far, unified decision about the plane of weakness and failure of borehole on shale is yet to be fully realized by the industry, the most common of these losses are: widening of the well, washouts, stuck pipes, cavings, and deformation of the casing, amongst others. This work highlights the effect of shale bedding plane failure on wellbore stability and the angle of attack for stable drilling conditions in weak bedding planes by developing a robust tool to account for the criterion/conditions for identifying and drilling weak bedding planes, the rock strength for weak bedding planes (Uniaxial Compressive Strength UCS), minimum mud pressure to prevent shear failure & wellbore slip or slip shear failure in weak bedding planes and suitable attack angle to the bedding planes was also developed. The proposed tool has increased drilling efficiency and has been validated with field data. The advantage of this process is reduction in drilling difficulties and non productive time (NPT) in field operations in the Niger Delta where well construction targeting deeper horizons are proving to be a challenge. The new rock strength for weak bedding planes (UCS) helps to predict the minimum mud pressure to prevent shear failure & wellbore slip or slip shear failure in weak bedding planes having determined the best angle to place the well (angle of attack).