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.

Wellbore instability all over the world accounts for a significant in Non-Productive Time (NPT) in well drilling and completions and incurring high cost of drilling as well as increasing safe risks. Real time analysis of data and geomechanics (understanding the rock properties and stresses) has continuously improved the stability of wellbores in the world. Some of the issues cost by this instability are the most common of these losses are: borehole enlargement, cavings, washouts, stuck pipes, deformation of the casing and amongst others. Numerous wellbore instability problems related to drilling through potentially natural fractured reservoirs/formations have been reported. Most of the this reported formations are been characterized by number of macro and micro scale bedding planes and/or networks of natural fractures which weakens the mechanical strength of the rock and the producibility of potential of the rock matrix. This term paper reviews instabilities issues in Naturally Fractured Reservoirs (NFR), natural fracture reservoir failure mechanism and highlights the effect of instabilities in NFR as well as proper well placement in the NFRs. This paper also looked at wellbore solution in permeable and impermeable formations, borehole failure analyses, drilling strategies to mitigate instabilities in naturally fractured reservoirs and other counter-measures dealing with wellbore instability. The review shows that well design, drilling fluid design, minimizing lateral vibrations of drill pipe and good drilling practices are critical in solving wellbore stabilities in naturally fractured reservoirs.

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.

Water in oil emulsion occurs at many stages in the production and treatment of crude oil. About two thirds of the produce of every new oil field exists in the form of water-in-oil emulsions. The emulsion stability results from the presence of interfacial barrier preventing coalescence of the dispersed water droplets; this is due to the present of polar components such as asphaltenes, resins, and wax and naphtenic acids in the crude oil. Therefore, before transporting or refining the oil, it is essential to separate the water for economic and operational reasons. In this research, three demulsifiers were used (Igepal co-720, Tween 40 and Tween 20) with the experimental work performed under room temperature. The results obtained with chemical demulsifier Igepal co-720 with Hydrophilic-Lipophilic Balance (HLB) of 14 were {31, 27, 24, 22 and 22%} of water separated from 5,4,3,2 and 1 ml concentration respectively at 60 to 120minutes optimum separation time. Similarly, from chemical demulsifier Tween 40 with HLB of 16 was {80, 74, 72, 70 and 55%) water separated from 5,4,3,2, and 1ml concentration respectively at 60 to 120minutes Optimum separation time. Finally, results obtained from chemical demulsifier Tween 20 with HLB of 17 were {95, 83, 67, 60 and 50%} respectively of water resolved from 5,4,3,2, and 1ml concentration at 60 to 120 optimum separation time. Comparing the three different chemical demulsifiers, it showed that Tween 20 has highest resolving power followed by Tween 40 and the least Igepal co-720. From this result it could be deduced that the higher HLB, the higher the separation of water in emulsion. In addition, a ranking of the demulsifiers used showed that highest capacity demulsifier was the demulsifier with highest HLB.

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).