Summary We present a novel approach that combines dynamic reservoir simulations and special core tests to model the extent of invasive damage and its impact on flowback during production. A radially adaptive 3D microsimulator is used to estimate the extent and impact of filtrate invasion on near-wellbore saturation and reservoir pressure. Time-varying reservoir exposure is used to simulate the acts of drilling, tripping, completions, and workovers. Extremely fine, core-scale grids are used to capture saturation and pressure in the invasion zone. Special core tests using a specially designed core holder are conducted on the subject reservoir core. Test results are interpreted to obtain an estimate of endpoint relative permeabilities, dynamic mudcake effect on filtrate loss, and impact of solids invasion on return permeability. The saturation and pressure profiles from this model are then used as initial conditions in a sector-scale simulator to model flowback effects. Absolute-permeability damage is modeled using the core-test results as an incremental and hyperbolically recovering effect during flowback simulations. A near-wellbore fine-grid overlay is used to capture the near-wellbore effects from the microsimulator results. Several sensitivities, including initial reservoir pressure, degree of overbalance and drawdown, heterogeneity, anisotropy, and mudcake effect, are examined. Equivalent skin factors that vary with time and depth are developed to enable comparison with full-field simulations. A horizontal-well example is used to illustrate the results of the study. Results illustrate the stark and often underappreciated effects of invasive damage on flowback and, therefore, on production performance. The methods described in this work can be used in reservoir-specific studies to quantify formation damage and aid in the selection of mud types, drilling techniques, and remediation methods required to improve performance. It is hoped that this work bridges the typically empirical damage-characterization methods and dynamic reservoir simulations. Introduction Conventional (or overbalanced) drilling and workover operations invariably result in invasion of filtrate and solids present in the drilling and workover fluids. In most cases, the damage caused is limited to a near-wellbore region and can reduce productivity because of degradation in effective permeability. Permeability degradation from filtrate and solids invasion could be caused by a variety of damage mechanisms, such as blockage of pore throats by solids, reduction in relative permeability to hydrocarbons because of a change in saturation, phase blockage, and clay swelling in the formation. Damage can be harsher in horizontal wells and mature reservoirs because of greater overbalance and longer duration of exposure to drilling fluids. During drilling, mudcake buildup can reduce the invasion depth. The buildup and effectiveness of mudcake depend greatly upon the formulation of the mud, the type and heterogeneity of the formation being drilled, the maturity of the reservoir, and the degree of overbalance during drilling or workovers. In horizontal wells, mudcake effectiveness is compromised further because of repeated movement of the pipe against the mudcake, leading to several events of removal and re-laying of the mudcake. The effects of damage also can be alleviated by the use of remedial stimulation techniques such as acidizing and hydraulic fracturing. These may not always produce the desired results, particularly in horizontal wells in highly heterogeneous formations. Moreover, implementing some of these techniques in horizontal wells is difficult. Given the potential for reduced productivity from invasion, characterization of invasion-induced damage has been of interest for decades. However, the implicit presumption when dealing with invasion-induced damage has been that it can be mitigated (by appropriate selection of muds and formation of mudcake), bypassed (through perforations), or remedied (through stimulation and fracturing). Most prior damage-characterization work has been empirical in nature, relying on log and core tests to assess damage parameters. More recently, some authors also have attempted to quantify and model formation damage from the fundamental principles of deep-bed filtration, fines migration, and percolation theory. Dynamic modeling of invasion with numerical simulations has also received much-needed attention in recent times. However, much of the numerical invasion-modeling work in the literature has focused on the invasion only (typically because of interest in the impact of the invasion zone on log accuracy), and very few works have dealt with the impact of invasion on flowback during production. The problem of bridging empirical models and dynamic simulations to obtain reasonable estimates of the impact on production has been one of the challenges. In this work, we present a novel approach that combines dynamic reservoir simulations and special core tests to model the extent of invasive damage and its impact on flowback during production. The approach uses an ultrafine-grid numerical simulator to model invasion, with parameters calibrated to special core tests. Flowback is then modeled using a sector-scale simulator with near-wellbore fine gridding, with the initial saturation and pressure profiles as determined by the invasion model and parameters calibrated to the core tests. The experimental and numerical approaches are described in detail, along with examples to illustrate the use of the methods we describe. Several sensitivity analyses are presented to demonstrate the often overlooked and underestimated impact of invasion on productivity. The method can be used to compare different mud types, evaluate the benefits of different remediation methods, and value the impact of underbalanced drilling (UBD) on productivity.

Shell is a major player in the Global deployment of Managed Pressure Drilling (MPD) technology to reduce drilling prob-lems, minimize formation damage and improve reservoir management techniques. The impetus is a recognition that most of the world's mature hydrocarbon reservoirs are in the lower end of the depletion cycle and an increasing number of horizontal wells are left with un-cemented completions. In addition, to access new untapped reserves, requires venturing into deeper water depths and the exploitation of deeper tight gas reservoirs, each with its own challenges. This paper describes the process involving rigorous pre-screening, planning, execution and feedback of learning de-veloped that enables fast implementation of underbalanced drilling (UBD) and integrated technologies into "Brown Field" areas and new business opportunities in "Green Field" devel-opment. This begs the question; what is Managed Pressure Drilling technology and what is the relationship with underbalanced drilling? The International Association of Drilling Contractors (IADC) defines Managed Pressure Drilling as "an adaptive drilling process used to precisely control the annular pressure profile throughout the well bore. The objectives are to establish the down-hole pressure environment limits and to manage the annular hydraulic pressure profile accordingly". In a sense managed pressure drilling, covers all drilling, since management of pressure is the goal of all drilling activities. However, there is general industry agreement that Managed Pressure Drilling is an umbrella term that refers to drilling activities conducted in a closed loop system. Conventional overbalanced drilling sits at one end of the Managed Pressure Drilling spectrum and underbalanced drilling at the other end. In conventional drilling, bottom hole pressure is managed by controlling the density of the drilling fluid to maintain well bore pressure profile above the pore-pressure throughout the well bore. Underbalanced drilling on the other hand, the well bore pressure profile is intentionally kept below the pore-pressure of all the exposed formations in the well bore. In be-tween these two extremes are various techniques used to con-trol the annular pressure profile and overcome constraints im-posed as a result of the equivalent circulating density of the mud system. Seen from Shell's perspective, Managed Pressure Drilling is a basket of drilling techniques that can be adapted to solve drilling related problems, reduce formation damage and or dynamically characterize production reservoirs (while drilling) to enable improved reservoir management. It is also worthy to note that not all Managed Pressure Drilling tech-niques require a closed loop system. Introduction Shell has deployed underbalanced drilling in oil and gas reser-voirs since the early 1990's. The introduction of horizontal drilling in the reservoir in the 1980's resulted in many new challenges. It created the need for a different approach to en-able drilling in depleted and fractured reservoirs. Shell initi-ated underbalanced drilling operations with foam1 in 1992 and in 1993 and 1994 the company conducted trials using multi-phase drilling fluids and the closed loop 4-phase separation system. In 1995, Shell successfully drilled the first horizontal underbalanced well using coiled tubing in Canada2. In 1997, Shell introduced underbalanced drilling in the offshore envi-ronment3. In the late 1990's, our Research and Development staff also developed and introduced expandable tubulars to the industry. A global approach to deploy these technologies was initiated in year 2000, when underbalanced drilling and ex-pandable tubulars were launched as two of four key technolo-gies (the others being 4D Seismic and Smart Wells) to be taken up within the Shell Group. Early on, the synergy be-tween underbalanced drilling and expandables was recognised as was the synergy between these two technologies and swel-lable elastomers and research and development work in these areas was supported.

TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractWe present a novel approach that combines dynamic reservoir simulations and special core tests to model the extent of invasive damage and its impact on flow-back during production. A radially adaptive 3D "micro" simulator is used to estimate the extent and impact of filtrate invasion on nearwellbore saturation and reservoir pressure. The lateral extent of the model is limited only to the invasive zone, and timevarying reservoir exposure is used to simulate the acts of drilling, tripping, completions and workovers. Extremely fine, core-scale grids are used to capture saturation and pressure in the invasion zone. Special core tests using a speciallydesigned core holder are conducted on the subject reservoir core. Test results are interpreted to obtain an estimate of endpoint relative permeabilities at irreducible saturation, dynamic mud cake effect on filtrate loss, and impact of solids invasion on return permeability. Both overbalanced and underbalanced tests are conducted with different drilling fluids. The tests are used to describe the mud cake effect in the invasion model. The saturation and pressure profiles from this model are then used as initial conditions in a sector-scale simulator with nearwellbore fine-gridding to model flow-back effects. Absolute permeability damage is modeled using the core test results as an incremental and hyperbolically recovering effect during flow-back simulations. A near-wellbore fine grid overlay is used to capture the near wellbore effects from the microsimulator results. Several sensitivities, including initial reservoir pressure, degree of overbalance (during drilling) and drawdown (during production), heterogeneity, anisotropy, and mud cake effect are examined. Equivalent skin factors that vary with time and depth are developed to enable comparison with full-field simulations. A horizontal well example is used to illustrate the results of the study. Results illustrate stark and often under-appreciated effects of invasive damage on flowback and therefore on production performance. The methods described in this work can be used in reservoir-specific studies to model formation damage and aid in the selection of mud types, drilling techniques and remediation methods required to improve performance. It is hoped that this work bridges the typically empirical damage characterization methods with dynamic reservoir simulations.

Underbalanced drilling is not a new technology. All wells drilled up until the introduction of the rotary drilling rig were drilled underbalanced. However, knowledge of subsurface conditions was limited, well control risks were not managed and well control incidents, when wells struck over-pressured zones, were common. The introduction of the rotary rig, resulted in mud circulation systems. As wells were drilled deeper into over-pressured formations blowout preventers (BOPs) that enabled better well control were introduced. Overbalance (conventional) drilling replaced "underbalance" cable tool drilling and became the normal practice. In conventional drilling, the hydrostatic pressure created by the mud column provides primary well control and BOPs provide secondary well control. Underbalance drilling has emerged in the past 15 years, and, just as there have been improvements and refinements in conventional well control equipment and procedures overtime, the same holds true for underbalanced drilling technology. In underbalanced drilling, the primary well control function of the mud column is replaced by a combination of flow and pressure control, while secondary well control functionality is provided by the BOPs. The complete UBD closed-loop flow control system comprises of the drill pipe (DP) circulating system, a UBD control device (UBD-CD), which provides the sealing mechanism around the drill string at surface, a UBD choke manifold (not the rig's well control choke manifold), a UBD separator and flare system. In addition, non-return valves (NRVs) are installed in the bottom hole assembly (BHA) and drill string to prevent flow up the DP. This paper provides an overview of underbalanced operations primary well control. It briefly describes the equipment and techniques used and the critical issues to consider during project planning to ensure the safety of staff, the rig, the well and to ensure that the life cycle objectives of UBD wells are addressed. Introduction Underbalanced drilling is not a new technology. Wells drilled in the early days of the oil and gas industry were drilled underbalanced with cable rigs. Cable rigs make hole by repeatedly dropping a heavy tool string onto the rock-cutting bit. The process stops every few feet and the bottom of the hole is cleaned with a bailing bucket. This technique, which is still in use today for drilling domestic water wells in rural areas, is safe, but hole stability and water influx results in some pretty slow rates of penetration.

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