PMCD

Abstract: In PMCD operations, reservoir gas is expected to migrate uphole, and the uncertainty in gas migration rates under downhole conditions leads to challenges in planning logistics and fluid requirements. Estimates of migration velocities based on current methods (e.g. Taylor-bubble correlation) are highly conservative and involves simplifying assumptions. This paper presents a systematic approach to understanding the fundamentals of gas migration in wellbores, relates it to field data, and provides recommendations to improve PMCD design and planning. Our approach includes analysis of PMCD field data, multiphase flow literature and computational flow simulations. The field data on gas migration is used to establish the field-scale parametric effects and observed trends. Multiphase flow literature is used to qualitatively understand some of these parametric effects at downhole conditions. A comparison between multiphase flow literature and field data overwhelmingly demonstrates the gaps in understanding of underlying physics. 3-dimensional multiphase CFD simulations for a representative well geometry and downhole conditions are used to understand gas migration physics at downhole conditions and the reasons for its sensitivity to different conditions. CFD simulations showed a strong impact of pressure on bubble breakup. As a result, the gas migrates as a slow-moving swarm of smaller bubbles. The formation of smaller bubbles from a given gas volume is a rate dependent process and requires a finite time to reach to an equilibrium/steady-state. The field conditions provide both high downhole pressure and sufficient length-scale for formation of smaller slow-moving bubbles. For the same reason, small scale-experiments are limited in their application for field-scale designs due to use of low pressure and/or insufficient length-scales. The CFD results also compare well with field data in showing ~30% holdup of migrating gas at low migration rates and negligible effect of rotation and wellbore geometry i.e. annulus vs openhole. The extent and rate of disintegration of gas volume (bubble) has a negative correlation with well inclination, liquid viscosity, and surface tension. The rheology and liquid viscosity also affect the ability of liquid to sweep the gas back into the reservoir and therefore it is expected to have an optimum range for a given PMCD application. Use of high viscosity fluids for typical downhole well conditions is counterproductive and results in higher gas migration rates and therefore not recommended. The understanding of downhole physics is expected to improve logistics/storage/ planning/fluid choice and lead to lower gas migration rates and reliable operation. The same approach can be applied to other operations and scenarios where gas migration velocities are a key design factor.

Abstract: Corneal pellucid marginal degeneration (PMCD) is an idiopathic condition characterized by non-inflammatory, non-ulcerative thinning of inferior peripheral cornea. PMCD has been reported occasionally complicated with hydrops owing to break in descemet membrane. We herein report a 38-year-old man, who presented with sudden dimness of vision in right eye. Clinical findings and Orbscan II were suggestive of PMCD in both eyes with hydrops in right eye. Slit-lamp and optical coherence tomography of right eye showed central descemet’s detachment without any break. Patient underwent descemetopexy by isoexpansile C3F8 (14%) and is doing well with significant improvement in the hydrops.

Abstract: A majority of the “easy” fields have already been developed, while demand for oil and gas continues to increase rapidly. Reservoirs in deep-set carbonates contains a large amount of the worlds remaining hydrocarbons and could pose as a solution for supplying the future demand. However, extracting these hydrocarbons has proven to be a daunting task. Carbonate formations are often severely fractured and karstified, leading to large or even total losses during drilling. As these fractures and “caves” are also the main target for gas, kicks and blow-outs are a constant threat. In this master thesis a variant of Managed Pressure Drilling (MPD), Pressurized Mud Cap Drilling (PMCD), has been reviewed. PMCD uses a static mud cap in the annulus to provide adequate downhole pressure, while a cheap sacrificial fluid is pumped down the drillstring to remove cuttings and transport it into the formation. A literature study was done in order to compare PMCD against other existing tech- niques, specifically conventional drilling and Constant Bottom Hole Pressure, another MPD variant. Working along side the losses have enabled PMCD to safely drill to Total Depth in these reservoirs, while reducing most of the Non-Productive Time and having an overall cheaper operation. Where other techniques are relying on time consuming and costly Lost Circulation Material, cement or other means of plugging the formation, PMCD works at its optimal. A static model was made to more clearly show the physics behind PMCD and to be able to simulate an operation through a gas bearing total loss ”cave” in a deep- set carbonate environment. Its procedures and advantages became clear, though the model and reservoir environment was rather simplistic. After an evaluation, the results were that PMCD lacks versatility and is not yet fully accepted by the industry, but that it offers the best solution for drilling of deep-set carbonates. The main conclusion is that in these reservoir, PMCD should as a minimum be used as a contingency in exploration wells.

Abstract: This paper describes the implementation of pressurized mud cap drilling (PMCD) technology, a variant of Managed Pressure Drilling (MPD), a successful technique frequently used on oil and gas fields in Kazakhstan. It also considers the planning phase, operational aspects, and results of drilling with the PMCD technique through challenging formations. PMCD technology with a rotating control device (RCD) is a form of blind drilling, where the drilling fluid and formation cuttings are not transported to the surface. It is a non-conventional drilling technique designed to maintain annular wellbore pressure to prevent total loss of circulation. A sacrificial fluid (SAC) is injected through the drill string and light annular fluid is pumped down from the annulus to maintain borehole fill and prevent annular gas migration. Wells in this field have encountered uncontrollable losses while drilling sections of the fractured carbonate. As a result, the application of PMCD technology to meet those challenges was an obvious choice in order to achieve target depth. Conventionally drilling of the 8-in. section resulted in fluid losses of more than 450 m3. Consequently, passing through these challenging zones the rig crew switched from conventional drilling to PMCD. The wells were then successfully drilled using the PMCD method, without any issues or well-control incidents, and planned TD was attained. By enabling the client to reach TD, Weatherford PMCD equipment transformed a previously undrillable well into a potentially valuable asset. This operation demonstrated that PMCD can be a viable drilling technique for future wells in the field. PMCD technologies included reduced consumption of lost-circulation material (LCM) and reduced loss of mud to the formation, keeping the wells economically viable. The main objectives of these wells were to drill safely and efficiently to target depth (TD), to deliver the wells for production on schedule, reduce non-productive time (NPT), minimize the drilling risks and hazards, and optimize the drilling program.