With the continued growth of drilling activities and the cost of performing them in depleted sands, loss of productive time is more important than ever. For years the use of LCM has been the preferred way for treating lost-circulation problems (Messenger 1981). However, the lost circulation problem was not totally resolved. Recent studies have shed much light on its cause and potential solutions. However, wellbore strengthening or stress cage implementation has been recognized as an effective means of dealing with lost circulation during drilling operations. One of the mechanisms developed for strengthening a wellbore has been to prop induced and existing fractures with particulate lost circulation materials (LCM) to effectively increase hoop stress in the near wellbore region. However, a good understanding of this mechanism is necessary in order to avoid a potentially flawed design and implementation process which could adversely affect job success during well operations. One of the main issues is depleted sand/shales stringers stability under the strengthening conditions. This paper will describe those factors which are important in designing wellbore strengthening jobs and address the conditions necessary to help ensure depleted sands stability, as determined through geomechanics analysis. The strengthening of a wellbore by propping fractures has been discussed in a previous investigation (Wang et al. 2007a). In this paper, various parameters that affect the strengthening of the wellbore are addressed in detail. In-depth discussion of how each of those parameters affects the process of wellbore strengthening will be presented. This study was accomplished using a boundary element numerical analysis coupled with the linear elastic theorem. Hence, the stress cage concept is an approach developed to enhance wellbore pressure containment (WPC). It has been found that a weak wellbore can contain much higher pressure if the wellbore fluid is treated with particulates and as such understanding the mechanism of stress caging is recommended to design the treatments for specific field application and to advance the development of the technology for application across a wider mud weight window and with a higher success rate while drilling in depleted sands. On-the-fly preventive methods appear to be favored as they cut down on non-productive time and reduce costs in the longer term while drilling in depleted sands as can be seen using the new 3D MUDSYSTTM model.

Saving time and money are the results of the stable bore hole design. During drilling there are two main instability problems, namely, bore hole collapse and fracture. The consequences of these drilling problems are severe, even the most simple bore hole collapse or break down can lead to the loss of millions of dollars in equipment and valuable natural resources. The main aspect of the well bore stability analysis is to mitigate these drilling problems. This is typically investigated by a constitutive model to estimate stress around the well bore, coupled with failure criterion. The objective of this study is to develop a model to widen the operating mud window by increasing the shear strength which involves reducing the minimum mudweight required to keep the hole open, and also increasing the near – wellbore compressive tangential stress which is coupled with the increase in maximum mudweight prior to fracture initiation. The most common approach for stability analysis is a linear elastic and isotropic constitutive model in conjunction with linear failure criteria like Mohr-Coulomb. The Mohr- Coulomb failure criterion only involves the maximum and minimum principle stresses and therefore assumes that the intermediate principle stress has no influence on rock strength. In addition, it is believed that the fluid barrier, and a part of the bore hole wall, behave plastically which provides higher fracturing pressure than conventional elastic theory. In this paper, a model for the mud weight window determination, using Mogi-Coulomb failure criterion and the elasto plastic model is developed. This is based on the hypothesis that, indeed elastic constitutive model does not fit with the reality of the well bore wall behavior and intermediate principle stress plays an important role on rock strength. This model leads to easily computed expression for the critical mud pressure required to maintain well bore stability and obtain a wider mudweight window for borehole strengthening during vertical and deviated well operations.

Mitigating drilling hazards ‘balancing drilling risks against the optimum well design while preparing for unplanned drilling events ‘has been a challenge to cost-effective well construction for decades. The pore pressure/formation fracture gradient balancing challenges mixed with the unexpected encounters with shallow flows, unstable formations, overpressure formations and depleted formations makes AFE (authorization for expenditures) goals dim. Excessive use of loss circulation pills and traditional contingency liners drive well costs up and jeopardize reaching total depth (TD) with an effective completion. A drilling hazard remediation solution could be as simple as using that planned contingency liner or using ddrrill-in casing to fight sloughing formations (McLean 1990). However, the use of conventional soolid expandable drilling liners can drive excessive risks into the well and even cause a costly sidetracking of the well on the drillers table. NPT in operations within areas of gulf of Guinea, gulf of Mexico and North sea have been successful due to absence of subsurface well integrity, lack of offset well information and economics. In view of this an Eco - geomechanical tool (GECDrill™) has been developed to handle the uncertainties created as a result of subsurface anomalies during well operations. Borehole drilling arises from the observation that an experienced driller can develop a good "feel" for the nature of formation being drilled. It should therefore be possible to establish an Eco - geomechanical tool (GECDrill™) to derive formation properties, subsurface geomechanical elements and cost indicators for economic evaluation through certain correlations. To minimize the NPT on drillers table, the Eco – geomechanical tool was developed based on realistic stress models coupled with linear elastic theorem and an economic risk assessment tool using a Monte Carlos bi-coupling strategy to evaluate the level and degree of NPT incurred while drilling in all fault scenarios.

Drilling fluids are vital elements in the safe, efficient and effective construction of wells. Their key functions include transporting drill cuttings to the surface, cooling and lubrication of drill string, cleaning build-up deposits on drill bits and tools, as well as stabilisation of the borehole and pressure control. Because they are often a complex mixture of different solids and fluids, the rheology of drilling fluids is usually complicated. As a result, they typically exhibit non-Newtonian flow behaviours. While the traditional practice is to use critical velocity to describe the flow regimes of drilling fluids by discriminating between laminar and turbulent conditions, this paper explores the applicability of Reynolds numlber (NRe), which is a more robust and universal dimensionless quantity to characterise flow regimes. Models to estimate NRe of drilling fluids are explored for Bingham and power-law types of drilling fluids, including development of models for other non-Newtonian behaviours such as shear-thinning and shear-thickening. More important, the models provide a veritable basis to compare the hydraulic characteristics of a drilling-fluid mixture against its Newtonian counterparts under similar conditions. In addition, these models would facilitate the exploitation of the concept of dynamic similarity to improve the design and benchmarking of the flow characteristics of different drilling fluids in different systems and under diverse conditions. Examples are provided that show the robustness of using NRe as against critical velocity, to identify flow regimes of drilling fluids. The applicability of the proposed models and ideas are not limited to drilling fluid hydraulics. The findings are relevant in other areas of transporting non-Newtonian fluids such as polymer for enhanced-oil recovery and multiphase mixtures such as emulsions, waxy crudes and general pipeline transport. Additionally, the principles and insights should be of interest to other industries such as food processing and chemical manufacturing.

First E&P commenced her development drilling campaign in the ALA field with a set of four batch wells (ALA west Phase 1A). Casing design and well architecture for the four wells were determined using standard design software, as the field had significant appraisal activities. However, actual drilling of the top-hole section on the four wells were very challenging such that, only in one of the four wells was the surface casing successfully run to depth and cemented as planned. Issues encountered included but not limited to: Trouble drilling through very reactive gumbo clay. Trouble pulling out of hole drilling BHA. Trouble running surface casing through long section of reactive clay causing stuck pipe (Casing got stuck while running in hole). Although, a leading contribution to getting stuck was differential sticking which can be attributed to not being able to run the casing with centralizers installed (bow spring centralizers) because they got hung up at the conductor shoe which was a buckled at the shoe due to piling effects. As a result, this study was carried out to investigate the possibility of mitigating the identified challenges, while optimizing the drilling of the top holes to improve casing and cementing operations. A starting point was to estimate the theoretical minimum casing setting depth. Using a Pressure Balance method, calculations were made to derive a mathematical model for the kick tolerance. The kick tolerance requirements were then derived in line with company policy and pore/fracture pressure information from offset wells and studies data (MDT & LOT) to arrive at the minimum casing setting depth. A second mathematical model based on limited gas kick model load case was also derived from pressure balance calculations, to estimate casing internal pressure profile when a gas bubble reaches surface during well control circulation using drillers method. A realistic criterion for estimating burst load, consistent with the definition of kick tolerance, was then proposed, to optimize the casing design. The study estimated cost savings of up to $1 million/well could be realized. The approach confirms huge cost savings can be realized by optimizing casing setting depths, and that illustrates the impact of safety factors. The depth proposals from this study were like the depths planned for the new drilling campaign of phase 1A+ wells, the result of actual drilling of the phase 1A+ wells (ALA-5 & ALA-6) confirmed the following, Surface holes in ALA field can be drilled to shallower depths. This may have shortened the time to run the casing to depth given historical challenges. The actual depths of the surface casings are like the depth proposed by this study. Problems of drilling with water-based mud can be mitigated if drilling intervals are short. A price comparison of top-hole actual drilling and running casing of ALA-3 a phase 1 well and ALA-5 & 6 phase 1A+ wells showed significant cost saving estimated above $1 million. This is like the cost saving estimated from the initial study. The surface hole drilled total depth and casing shoe depth on the phase 1A+ wells confirmed the pre-drill kick tolerance estimated in the study for these depths in the phase 1A+ wells.