TX 75083-3836, U.S.A., fax 01-972-952-9435. AbstractPoor gas inflow performance is observed in some depleted, low permeability, reservoirs after completion and workover operations. The use of aqueous treatment fluids often results in a 'water block' due to poor recovery of the fluids that have leaked-off. This curtails well deliverability due to reduced relative permeability to gas/oil in the invaded region.This study analyzes the effect of various factors governing the cleanup of water blocks in fractured and unfractured wells for both gas and oil reservoirs. The effects of drawdown, capillary pressure, relative permeability, and heterogeneity as well as the influence of fracture geometry on well deliverability following some well operations, such as fracturing, have been examined by detailed simulations.Drawdown, fracture length and shapes of relative permeability curves strongly affect the recovery in productivity. On the other hand, end point relative permeabilities and horizontal well length have an insignificant impact on cleanup. Higher vertical permeabilities favor early recovery of well productivity in 'high perm' layers and delay cleanup of water blocks in 'low perm' layers.The results suggest the need to lower capillary pressure by reducing interfacial tension and/or altering wettability of the rock surface from strongly water-wet to intermediatewet. With the correct selection of treatment fluids, proper design of fracture geometry and optimum drawdown applied it is possible to cleanup water blocks more rapidly in depleted lowpermeability reservoirs.

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Establishing connectivity among various injectors and producers is a key to improving the understanding of a reservoir under waterflood. This understanding improves the estimates for ultimate recovery and also helps to better define the future development plan. In deepwater turbidite reservoirs, numerical flow-simulation models are used to make performance predictions, with reservoir connectivity as one of the key uncertainties. In the initial phase of field development, interwell tracers were used to assess the connectivity. As more wells were drilled, updates were required for the simulation models. Instead of waiting for the next phase of an ongoing tracer program, both rate-transient analysis (RTA) and capacitance-resistance model (CRM) were used to understand connectivity. The input for both RTA and CRM are the rates and pressures, which are being gathered with real-time surveillance. This paper presents a case study to compare findings from the use of interwell tracer data with the results of CRM based on dynamic data. Another study element demonstrates the use of RTA in identifying and estimating the volume of thief zone. Attempts are made to use CRM and RTA to predict connectivity based on performance prior to experiencing water breakthrough. These case studies demonstrate the application of RTA and CRM in ongoing waterfloods. The CRM concurred with the initial tracer results and helped to understand the change in pressure distribution with time as the field was being developed. We learned that the use of CRM can be a viable alternative to an interwell tracer program to reduce uncertainty related to injector-producer connectivity. CRM also helped in understanding the efficiency of the injectors, which is important in a facility with limited water injection capacity. The ease of use of CRM and RTA makes them useful as screening tools in the process of developing a detailed flow-simulation model.

Material-balance (MB) analysis for in-place volume estimation in gas reservoirs has been in practice for decades. Nonlinear responses from geopressure reservoirs with or without aquifer influx present special interpretation challenges. One of the main challenges of in-place volume estimates involves the estimation of average-reservoir pressure with production. To that end, modern pressure sensors installed at bottomhole and/or surface largely help establish a given well's dynamic performance by way of rate-transient analysis.This paper explores the applicability and limitations of the standard analytical tools in volumetric, geopressure, and waterdrive systems for a diverse array of fluids, from dry gas to near-critical gas/ condensate. The systematic approach presented in this paper attempts to increase accuracy in results by ensuring consistency in solutions from multiple methods used to first assess the average-reservoir pressure from production performance data, followed by in-place volume estimation. In this context, we examined analytical tools, such as the p av /z vs. cumulative gas production (G p ) plot, and cumulative reservoir voidage vs. cumulative total expansion plot. Both pot aquifer and unsteady-state Carter-Tracy aquifer models were considered to account for water influx. Besides the use of Cole and drive indices plots, two diagnostic log-log plots are introduced involving total expansivity and change in average-reservoir pressure. In addition, we sought solution objectivity by introducing a diagnostic tool in the Walsh and Yildiz-McEwen MB plots. Both MB methods involve plotting of cumulative reservoir voidage (F) vs. cumulative total expansion (E t ), whereas the diagnostic tool consists of plotting F/E t vs. E t on the same graph.Initially, synthetic data helped us understand the overall system behavior and instilled confidence in the solutions obtained for various combinations of drive mechanisms. Statistical design of experiments prompted us to explore independent variables, such as aquifer-to-hydrocarbon PV ratio, production rate, degree of overpressure, and the aquifer source. Those learnings were validated with published and new field data encompassing an array of reservoirs with various drive mechanisms and fluid type.