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This paper presents a powerful approximate method for modelling the steady single-phase flow into a horizontal well completed with an Inflow Control Device (ICD) in an anisotropic reservoir. Two types of problems are investigated: the forward problem, which allows the user to find the flux distribution along the wellbore for a specified pressure drawdown, and the inverse problem to determine the ICD properties when the flux or reservoir pressure drawdown along the wellbore is given. The method is based on structuring the flow patterns around and, inside the wellbore and across the ICD and on the reduction of the dimensionality of the problem by using boundary integral equations. The resulting one dimensional singular nonlinear integro-differential equation is solved numerically, using the appropriate quadrature formula for singular integrals with Cauchy kernels.

A new methodology for evaluating the risk of gun jump during post-perforating surge is proposed and tested against field data. This methodology is corroborated by an advanced mathematical model of transient flow into the wellbore triggered by the creation of perforation tunnels under static underbalance and fully coupled with evolving transient flow inside the wellbore containing the gun string. The model predicts the viscous drag on the tool string and the gun string dynamics, which may induce cable tangling or breaking owing to gun jump. This model is used to mitigate the risk of gun jump by varying perforating job design options such as shot density, thickness of the perforated interval, and height of the liquid cushion. Examples of model application to the analysis of field data from Indonesia are provided. Introduction There are two basic mechanisms of gun jump during perforating underbalanced.1 One is associated with the dynamic impact of the shock wave created after initiation of the shaped charges on the gun string inside the borehole. This shock wave provides the gun string with momentum that can force it to move. This mechanism of gun jump can be activated perforating underbalance or overbalance and may result in both upward and downward gun movement, which is usually mitigated to some extent by shock absorbers included in gun strings. This mechanism of gun jump is not considered in the current study. The other mechanism of gun jump is related to the postperforating surge from the reservoir into the wellbore through perforation tunnels. If the underbalance pressure is high, the transient reservoir response coupled with the dynamic level of liquid inside the wellbore may create fast wellbore fill-up accompanied by viscous drag on the gun string and cable, causing the gun string to move upward.2 The more powerful the surge, the higher the risk of gun jump owing to viscous drag on cable. The risk of gun jump can be mitigated by reducing the static pressure underbalance, shot density, and thickness of the perforated interval. All these measures, however, can compromise the quality of perforation cleanup and the targeted well productivity. A reliable and quantitative technique is therefore needed to optimize perforating design, in particular, to choose the optimum cushion level, which would mitigate the risk of gun jump while providing maximum cleanup efficiency. A procedure for gun jump risk assessment based on a novel gun jump model is presented here. The proposed technique estimates the viscous drag on the cable by simulating transient surge from the reservoir coupled with wellbore fill-up. The paper is organized as follows. The surge model coupled with transient flow inside the wellbore is considered first using the approach developed in slug test and drillstem test theory.3, 4 The coupling takes into account the acceleration of the liquid column inside the wellbore, frictional pressure losses, and the variation of drawdown pressure to accurately calculate the drag force on the tool string. The drag force varies with time during surge to represent the input for a simple gun jump model that treats a gun on a cable as a mass on an elastic spring experiencing forced oscillations caused by varying cable tension. This drag force is compared with the critical drag force on the cable obtained from the condition that the cable tension becomes equal to zero when the gun reaches its highest position inside the wellbore. Surge Flow Physics The reservoir surge starts immediately after perforating with static pressure underbalance and continues until the equalization of pressure between the wellbore and the reservoir is achieved. The transient reservoir response during surge is coupled with the borehole fluid dynamics and controlled by different physical mechanisms during its evolution in time. As soon as the perforation tunnels have been created as shown in Fig. 1, the reservoir is in hydraulic communication with the wellbore, which is filled by the completion fluid up to an appropriate level with an air cushion above it. The initial size of liquid column inside the wellbore 0 L, or the size of the air cushion 0 H, is the main parameter that controls the static pressure underbalance during underbalanced perforating.

A novel concept for cleanup prediction during sampling is proposed. This concept shifts focus to the early phase of cleanup, allowing for early predictions of pumpout time or produced volume using optical fluid analysis logs versus given contamination targets. The early phase of cleanup potentially provides a great deal of information to be used in prediction of cleanup, because this phase is strongly affected by both the local flow pattern and the contamination transport and deals with a larger range of optical density variation. A new approach to cleanup prediction is based on a truly 3D model of flow and contamination transport to the probe production area at the wellbore wall covered by mudcake. This model better captures the initial phase of cleanup than the conventional spherical flow model, which incorporates axisymmetrical contamination transport to a small production sphere located at the wellbore axis. The new model provides the signature of 3D contamination transport on cleanup dynamics, which is controlled by the ratio of invasion depth to wellbore radius. The analysis of new problem solutions reveals new details of cleanup evolution. In particular, the transition from a predominantly circumferential regime of cleanup to a predominantly vertical cleanup has a distinctive signature that can be used in cleanup progress monitoring and the reconstruction of initial invasion depth. Examples of sampling job data processing that support the new concept are provided. They indicate that decent estimates of pumpout volumes can be obtained 3 to 5 times earlier using the new approach. Introduction Formation fluid sampling by the wireline formation tester (WFT) during drilling operation represents an important component of the formation evaluation system established by the petroleum industry (Fundamentals of Formation Testing, 2006), especially when it deals with high-profile and offshore wells. It is well known that the errors in estimates of formation fluid properties can lead to significant miscalculations in design and performance prediction of flow assurance, well construction, and production facilities. The main challenge in obtaining representative samples of formation fluid by WFT is related to the mud filtrate invasion during drilling. After a few hours of drilling, the borehole is usually surrounded by the invasion zone saturated predominantly with mud filtrate and, for this reason, any sampling operation launched during interruption of drilling has to start from the cleanup production, which continues until the target of contamination tolerance is reached or the time allocated for the sampling operation has run out. The major challenge of cleanup production monitoring represents the case of drilling with oil-based mud (OBM) due to miscibility of OBM filtrate with formation hydrocarbons and poor resistivity contrast. The variation of formation fluid properties during cleanup production, however, can be reliably detected by optics (Mullins, 2000). The existing optical fluid analyzers (OFA) can measure the optical density (OD) of produced mixtures in a wide spectrum of invisible light with the wave lengths in the range from 400 nm to 2,200 nm. In presence of initial OD contrast between the OBM filtrate and the formation fluid, the high sensitivity of optical measurements to the composition of produced fluid can be observed. This sensitivity, however, does not allow for the quantification of contamination in the produced fluid, since the OD of virgin formation fluid is unknown in advance. To compensate for the lack of information during cleanup production monitoring, the contamination transport prediction has to be involved to achieve the closure of the optical monitoring model.

This paper takes a novel approach towards designing and managing the architecture and operating protocol of injection/production system. The shut-in valve positioning and timing of valve closure control the amplitude and frequency of pressure waves generated during shutdowns. The proposed approach provides the means for mitigating the negative impact of water hammer on the integrity of the near wellbore region and provides an idea of the intensity of any cross-flow issues. It is based on a comprehensive model that examines the fast wellbore transients (water hammer) generated by routine or emergency shutdown of injector or producer wells, which can also cause interaction with a near wellbore region of reservoir. The modeling handles the coupling of the conventional transient pipe flow hydraulics with the transient reservoir flow. The decompression wave created by shutting down an injector interacts with the near wellbore region that induces a transient flow back from the reservoir which creates a risk of mechanical damage by dislodging and transporting material with the fluid movement and even result in sand production. The compression wave created by shutting down a producer may induce repeated injection pulses. In both producing and injection cases, multiple cross-flow phenomena can be triggered between formation layers and wells interconnected within the injection or production system. The analyses of these transient phenomena help to potentially quantify the mechanical damage, which may be induced in near wellbore reservoir region, and assess the potential damage risk associated with produced solids. IntroductionThe effect of water hammer on sand production in water injection wells and its detrimental impact on injectivity has been discussed in multiple publications 1-9 . The basic observation supported by field studies is that the bottomhole pressure drop induced by a pump shutdown or a valve closure can cause formation failure generating loose material. These solids can be produced and transported into the wellbore during bottomhole pressure oscillations associated with the water hammer pressure pulse's round trips. Larger solid particles usually settle inside the rat hole (sump) but small particles stay suspended with the fluid movement 2, 8 . Therefore these solids can induce formation damage due to natural or forced cross-flow between formation layers where injectivity contrast exists or during reestablishment of injection. Other potential damage mechanisms include the perforation tunnel collapse, the loss of rock cohesion near wellbore followed by the injectivity reduction, and the between well cross flow with transport of produced solids between injectors through manifolds or pipeline 5 .The schematic of fast transients responsible for cross-flow and formation damage in water injection system under subsea environment is shown in Figure 1. The system involves a riser (1-2), a manifold (2) connected with two injectors B01 and B02. In the case of FPSO pump shutdown, the water hammer manifestations may be ...