Andrew K. Wojtanowicz

Segregated production method for oil wells with active water coning

Abstract This paper describes a simulation study of a novel method to control water coning in an oil producing well. The method uses a dual-completion configuration that is above and below OWC. In this configuration, the well section above OWC is completed in the oil zone and produces oil, while the well section completed below OWC in the water-saturated zone produces water both independently of and concurrently with oil production. Segregated water production creates a drain (rat hole water sink) to control the rise of the water cone. Coning control results from the effect of water sink on flow potential between the sink and oil-producing perforations. The study investigated simulated production performance of the well both with and without the rat hole water sink. At first, the water sinks position and flow rate were examined to determine the critical production rate of oil (i.e., maximum production rate without the water breakthrough). Next, the various amounts of water produced during oil production at rates greater than critical were compared. The comparison study used both lab data and field data from an actual well/reservoir system of known geometry and reservoir properties. The coning behavior of the production system with the rat hole water sink was mathematically modelled using the flow potential distribution generated by two constant-terminal-rate sinks located between the two linear boundaries and the constant-pressure outer radial boundary. The model was verified by using the existing and simulated data of water coning in conventional wells and by setting the water sink flow rate at zero, with oil being the only produced fluid. Also, verification tests were used to select the type of oil reservoir with the thick oil zone and the small dip angle for the comparison studies. The study demonstrates the existence of an active mechanism to control water coning with the rat hole water sink. It shows that a sink with low water flow rate may prevent the water cone from breaking through the oil zone and into the producing perforations so that the oil with no water cut can be produced above the critical flow rate. A simulation using the new method yielded less produced water and either the same or a higher oil rate than the conventional method. However, potential increase of oil production above its critical value is limited by the stability of the controlled water cone. Also, for high oil production rates, more precision is needed to control the water production rate.

Laboratory Study of Borehole Friction Factor With a Dynamic-Filtration Apparatus

This paper discusses a medium-scale borehole friction tester with dynamic-filtration capabilities that showed that friction coefficients are affected by mud quality, mudcake, and lubricant addition in low-solids, water-based muds (WBMs). Most measured values, including those from oil-based muds (OBMs), were between 0.2 and 0.3. This paper also reports on new effects of filter cake and solids deposition on borehole friction.

Mathematical Modeling of PDC Bit Drilling Process Based on a Single-Cutter Mechanics

An analytical development of a new mechanistic drilling model for polycrystalline diamond compact (PDC) bits is presented. The derivation accounts for static balance of forces acting on a single PDC cutter and is based on assumed similarity between bit and cutter. The model is fully explicit with physical meanings given to all constants and functions. Three equations constitute the mathematical model: torque, drilling rate, and bit life. The equations comprise cutter’s geometry, rock properties drilling parameters, and four empirical constants. The constants are used to match the model to a PDC drilling process. Also presented are qualitative and predictive verifications of the model. Qualitative verification shows that the model’s response to drilling process variables is similar to the behavior of full-size PDC bits. However, accuracy of the model’s predictions of PDC bit performance is limited primarily by imprecision of bit-dull evaluation. The verification study is based upon the reported laboratory drilling and field drilling tests as well as field data collected by the authors.

Progression of injectivity damage with oily waste water in linear flow

Numerous laboratory experiments and field cases show that even very small amount of oil in injected water can cause severe injectivity damage. Although injectivity decline caused by oil droplets has been studied experimentally, there is still lack of an easy-to-use and widely accepted model to predict the decline behavior. In this work, we developed an analytical model to predict the time-dependent progress of the water permeability reduction in linear flow by analyzing experimental data obtained from linear core flooding.The model considers mass transfer of the oil phase from the produced water to the rock due capture effects by dispersion, advection and adsorption inside the rock. As the captured oil saturation increases, permeability reduces following the relative permeability drainage relationship. The reduction stabilizes when the oil saturation comes to an equilibrium value controlled by oil droplet size and injection velocity. The model is calibrated using published experimental data from prolonged core floods with oil-contaminated waste water. Theoretical runs of the model replicate all the effects known from experimental observations. Resulting from the model is a distributed change of permeability vs. time and distance from the point of injection that can be converted to the overall injectivity damage.

Numerical Modeling of the Effects of Disproportionate Permeability Reduction Water-Shutoff Treatments on Water Coning

In the present paper, we analyze numerically the disproportionate permeability reduction (DPR) water-shutoff (WSO) treatments in oil production well, i.e., the ability to reduce relative permeability (RP) to water more than to oil. The technique consists of bullhead injection of polymer solutions (gelant) into the near-wellbore formation without zone isolation. By assuming the low dissolution of polymer in oil and the low mobility of the gel in porous medium, we reduced the compositional model of the process to a simple twophase model, with RP and capillary pressure (PC) dependent on the water and gel saturation. We proposed the extension of the LET correlations used to calculate RP and PC for the case of three phases (oil–water–gel). The problem is divided into two stages: the polymer injection and the post-treatment production. Both of these processes are described by the same formal mathematical model, which results from incompressible two-phase flow equations formulated in terms of normalized saturation and global pressure. The thermal effects caused by the injection of a relatively cold aqueous solution are taken into account. The numerical solution shows favorable results for the DPR WSO treatments. Other techniques, such as the creation of impermeable barrier and downhole water sink (DWS) technology, are also tested in order to check the validity of the developed numerical model with experimental data. [DOI: 10.1115/1.4007913]

Downhole Water Sink (DWS) Completion Enchance OIL Recovery in Reservoirs with Water Coning Problem

To date, field trials of wells with DWS completions have shown that this new technology could control water coning and increase oil production rate. None of these tests, however, was long enough to show DWS potential to improve process of oil recovery comparing to conventional wells. Presented in this paper are results from a recent project of the RD A five-fold increase of the oil production rate was reached without changing the rate at the top completion - by adjusting the drainage rate at the bottom completion. The results also showed a 70-percent increase of oil recovery; from 0.521 up to 0.882 for conventional and DWS completions, respectively. The computer-simulated experiments with commercial reservoir simulator confirmed the thesis on better recovery with the DWS completions by showing a 30-percent increase of recovery factor from 0.61 to 0.79 for conventional and DWS completions, respectively. The results also gave a five-fold reduction of the time required to reach the limiting 0.98 value of water cut. However, accelerated recovery process with DWS required a substantial up to 3.5-fold increase of total water production. The simulation experiments clearly showed that the main advantage of using DWS is its flexibility of controlling the recovery process. For conventional completions, the recovery could be slightly increased by reducing production rates and largely increasing production times. For DWS, on the other hand, production process could be optimized for maximum recovery, minimum time, or minimum cumulative water produced by seeking a best combination of the top and bottom rates.

A field method for assessing borehole friction for directional well casing

Abstract A field procedure to evaluate the borehole friction factor between a pipe string and a borehole was developed. The primary concern of this research was to investigate and mathematically describe the drag associated with casing runs in directional wells through the use of the two-dimensional approach. The borehole friction factor was found, through an iterative procedure, by matching the calculated hook load to the measured one at the surface. The borehole friction factor was calculated as the unique value for the match. This study shows the necessary precision requirements for the hook load measurement instrumentation and presents three different types of equipment that meet accuracy demands. The significance of the mechanical friction effect on hook load was also addressed, by introducing the drag-weight ratio concept. It was found out that to assure accuracy of the method, the minimum required value of the drag-weight ratio should be 25%. The statistical analysis revealed that one single value of the borehole friction factor can represent the well. It was concluded in the study that the model can be used for prediction of tensional load of a casing string in a directional well.

Optimization of Large Gas Pipeline Network--A Case Study in China

In China, annual natural gas consumption is over 67 billion cubic metres with an expected growth rate of 10% per year. Most of the gas is transported from well heads to markets over cross-country gas networks, which requires construction of the West to East Gas Network - one of the largest gas networks in the world. Presently, the network is comprised of four large-diameter pipelines and will include most major gas pipelines in China in the future. The network distributes approximately 30 x 10 9 m 3 gas per year, of which 3% to 5% is burned to power the gas transportation. At current gas prices, gas transportation costs are roughly USD 350 million per year, which is a considerable cost that could be reduced by improvements in network design and operation. This paper reports on a study aimed at optimizing the network to minimize its energy consumption and cost. The large size and complex geometry of the network required breaking the study down into simple components, optimizing operation of the components locally, re-combining the optimized components into the network and optimizing the network globally. This four-step approach employed four different optimization methods, penalty function method, pattern search, enumeration and non-sequential dynamic programming, to solve the problem. The results show that cost savings, because of global optimization, are reduced with increased throughput. For example, increasing the gas rate from 67 - 90 million m 3 /d would reduce operational cost savings because of optimization from 23% -1.15%. Moreover, the study shows that if the compressors were fully loaded at their maximum rating, the optimized operation would approach the one being presently practiced. Thus, the optimization is effective and much needed when the system does not work at its maximum capacity, a typical case in the present operations of Chinese gas networks.

Cumulative Bioluminescence - A Potential Rapid Test of Drilling Fluid Toxicity: Development Study

A new rapid test of drilling fluid toxicity is based on the spontaneous bioluminescence of Pyrocystis lunula, an easy-to-culture alga that vigorously responds to shear stress (mixing) by emitting a sharp burst of light. In contrast to other bioluminescence methods, a cumulative flux of light is measured with a photomultiplier that eliminates the effect of exposure time on test results. Light quenching, caused by the presence of a toxicant, results in the dose/response relationship (DSR) typical for the enzymatic reaction kinetics. The Michaelis-Menten (dissociation) constant is used as a direct measure of toxicity. The evaluation study involved multiple experiments with 60 samples of drilling fluids from the U.S. gulf coast, as well as such typical toxicants as diesel oil, mineral oil, and chrome lignosulfonate (CLS). In this paper, the results of the test error analysis and comparisons with the Microtox and Mysid shrimp assays are reported.

Design of Cement Pulsation Treatment in Gas Wells-Model and Field Validation

Top cement pulsation (TCP) is an auxiliary cementing technology for enhancing zonal isolation by applying low-frequency hydraulic pressure pulses to the top of the wells annulus immediately after placing cement in the annulus. A properly designed TCP keeps the well overbalanced by delaying the process of cement slurry thickening in the well annulus without pressurizing the wells bottom. As a result, the cement slurry prolongs its liquid state, the thickening time is delayed, and transition time is shortened. The combination of these effects improve cement quality and eliminate gas flow after cementing. Using a hydraulic analogy between low-frequency reciprocation of Bingham fluid and the plug flow, a mathematical model describing the TCP treatment has been developed. The model employs a new formula describing plug flow pressure loss for the pulsed cement slurry. Also, equations have been developed to calculate the compressibility of the well annulus filled with cement and drilling mud. The TCP mathematical model calculates downhole transmission of the top displacement amplitude and pressure attenuation. The model gives the basis for TCP treatment design and efficiency prediction for a given well program and slurry properties. Also presented is an experimental verification of the TCP design model using full-scale pulsation of thixotropic slurry at the LSU well facility. Further verification is given by the field data from TCP treatments of cements in instrumented wells equipped with pressure and temperature sensors downhole.