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An Optimization Model to Determine Horizontal Well Location and Parameters for Strong Bottom Water Reservoir in Sudan
Abstract
The purpose of this study is to develop a horizontal well closed-looped optimization model for enhancing ultimate recovery of strong bottom water reservoir in Sudan.
An optimization model for horizontal well using the statistical properties of geology and development data, numerical simulation and fuzzy mathematics, is presented. The study was carried out as follows: 1) a mechanism model was established based on physical and fluid properties of the oilfields in Block 1/2/4, Sudan. The factors affecting the development performance of horizontal well, such as reservoir and fluid properties, inter-beds distribution, parameters of horizontal segment and production rate were discussed. 2) Development performance of typical massive bottom water reservoir in Sudan was analyzed. 3) The weight of each factor was determined using fuzzy mathematics method based on the mechanism study and on site practice of horizontal wells. 4) Determine the optimized location and production parameters of horizontal well by combining numerical simulation and integrated evaluation model.
The optimization model was used in H oilfield of Block 1/2/4, Sudan, the result of numerical simulation show that this model can efficiently determine the location of horizontal well and can lead to increase oil production in bottom water reservoir. It can also give a reference for numerical simulation, and have a great improve place.
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The present paper used the Lorenz curve to evaluate the heterogeneity of reservoirs and obtained permeability distributions of different heterogeneous reservoirs with a mean value of uniform formation conductivity by calculating the Lorenz curve inversely. A typical geological model for reservoirs with bottom water was established, and the flooding law of heterogeneous reservoirs with bottom water was studied by means of numerical simulation. The results indicated that when the variation coefficient (Vk) was lower than 0.2, the reservoir could be regarded as a homogeneous one, where bottom water was moving along high permeable zones and the permeability profile, flooding profile and liquid-producing profile were consistent with each other. There existed a modified logistic function between the breakthrough recovery and variation coefficient. The curve of variations between water cut and oil recovery in the semi-logarithmic coordinate system was in an S-like form and there was a function equation between water cut and the produced degree of recoverable reserves. When Vk < 0.3, the flooding mode was a linear breakthrough with flooding in the whole horizontal plane; when 0.3 ≤ Vk < 0.7, the flooding mode became to a punctiform breakthrough with flooding in the whole horizontal plane; and when Vk ≥ 0.7, the flooding mode turned to punctiform breakthrough with flooding in the local horizontal plane.
Recently, most technological advances in the petroleum industry have been in the areas of horizontal wells. Horizontal wells have been reported to produce up to 10 times more oil than that produced by vertical wells. Even though recent advances in horizontal well technology have led to a large drop in drilling and operation costs, very little has been done to advance mathematical modeling of a horizontal well in order to achieve more realistic prediction capabilities. This paper describes a reservoir simulator capable of dealing with important features of a horizontal wells, including weilbore dynamics and near weilbore radial flow.
In view of poor studying water-out pattern and laws in horizontal wells, a typical model is established on the basis of the geological data of a certain block in Jidong Oilfield. Through the study on water cut and water-oil ratio derivative curve on the typical water-out pattern, several conclusion are obtained: 1. One-point water-out; for heel water out, the water cut and water oil ratio derivative curves(WWDC) are smooth with two obvious water out points; for the middle water out, there are three water out points in the WWDC. As the high permeable zone moves to the toe, the duration of water free production becomes longer. 2. Multi-point water-out, the number of peaks on the WWDC grows with the increasing of the water out point. 3. Linear water out, there are obvious plateaus on the WWDC. 4. The water free production period of one point water-out pattern is longer than that of the two patterns. A new method is established to study the water out point by using production data.
The risks for development of high water-cut reservoirs in horizontal well were analyzed. The numerical simulation of watered-out mechanism in horizontal well was carried out. The basic models for describing the distribution of remaining oil in the well pattern of the injector-producer rows were established. The factors affecting the development behavior and production performance of horizontal well, such as reservoir scale, reservoir properties, fluid properties, parameters of horizontal segment, liquid rate and the distribution of interbeds were discussed. Sixty six cases and their sensitivity analysis projects were designed. The relative sensitivity of each factor to the production performance of horizontal well was investigated synthetically.
The waterflooding performance in different horizontal wells has significant difference. There are many affecting factors for water-flooding performance. On the basis of the production data of horizontal wells in bottom water reservoir in TZ402CIII formation of Tarim Oilfield and numerical simulation for production history matching, the waterflooding performance of horizontal well and its influencing factors were discussed. The waterflooding patterns of horizontal wells in bottom water reservoir of Tarim Oilfield were proposed, including linear breakthrough waterflooding in the whole horizontal section, punctiform breakthrough waterflooding in the whole horizontal section, and punctiform breakthrough waterflooding in the local horizontal section. This classification would be useful to qualitatively design of horizontal well, selection of the completion method and treatment methodology of horizontal wells in late period.
A 2D visible physical model of horizontal well production was set up using the advanced electronically monitoring technology and flow testing tools to investigate the fluid flow in bottom water driving reservoirs during horizontal well production. The formation and development of water coning were visualized, and the effect of water coning on water breakthrough times and recovery ratios was studied. When horizontal section is short, the water coning is formed in a short time, the two shoulders of the coning are steep and the oil/water interface deforms greatly. When horizontal section is long, the water coning is formed in a long time, the two shoulders of the coning are gentle and distributed symmetrically. With the increase of horizontal section length, the water breakthrough time delays, the water free recovery and the ultimate recovery increase. The increase of producing pressure shortens the production period before water breakthrough, and decreases both water free recovery and ultimate recovery.
Numerical simulation was used to determine the sensitivity of water coning behavior to various reservoir parameters. The simulation results were then used to develop a simplified correlation for water coning predictions. The correlation is general since the sensitivity studies cover a wide range of reservoir parameters (ratio of vertical to horizontal permeability from 0.01 to 1.0; perforated interval from 20 percent to 80 percent of oil zone thickness; production rate from 500 to 2000 RB/D gross fluid; mobility ratio from 1.0 to 10). The correlation is valid to predict watercut performance for most reservoirs with strong bottom water drives except those with local barriers present, those with a high degree of stratification (Dykstra-Parsons Permeability Variation > 0.8), or those with a thick oil-water transition zone.
With the correlation programmed on a hand-held calculator, field engineers can conveniently predict critical rate, breakthrough time and watercut performance. Compared to complicated numerical models, the correlation is particularly useful when detailed reservoir data are not available, or when decision time and project cost are limited. Field engineers can use the simplified correlation to compare watercut performance for various operating strategies and make appropriate decisions for production operations.
Cresting towards horizontal wells with bottom water drive and edge water drive has been experimentally investigated using a laboratory model. For bottom water drive the experimental results are compared with two models for transient crestal behavior. The range of applicability for critical rate estimates by the analytic model is found.
For edge water drive the model developed predicts the measured steady-state supercritical production performance over a wide range of flow rates. As a consequence of the model and experimental results follows that by increasing the total production, net oil rates much above the critical may be obtained.
Water coning is the mechanism in which the oil/gas – water contact locally rises toward the perforated interval in a partially penetrated oil/gas well.
Numerical and physical models have been built to study the performance of water coning under different boundary conditions. A fully implicit, strongly coupled mathematical model was formulated to handle rapid pressure-saturation changes. On the other hand, a plexiglass model was constructed to obtain a qualitative and quantitative description of water-coning phenomenon. An analytical description of water coning under different flow conditions was developed.
Water cones largely dictate oil recovery in a typical bottom-water situation. Major parameters influencing coning include mobility ratio, producing rate, completion interval, and anisotropy. We conducted a systematic study to understand the influence of these and other parameters on coning. Our aim was to develop producing and completion rules for managing the Greater Burgan’s Third Middle Sand (3MS) reservoir in Kuwait.
The building blocks of our study were: (1) detailed geologic modeling using core and openhole log data, (2) geostatistical modeling for 3D sector modeling, (3) transient-pressure testing, (4) production logs, and (5) cased-hole saturation logs. Singlewell, 2D radial models were built to understand various elements of coning for existing completions. We also explored alternative completions to mitigate coning. The study included the use of single- and dual-lateral wells and cone-reversal techniques. Follow up studies using a 3D-sector model confirmed some of the findings of single-well modeling.
Results show that high anisotropy provides near piston-like displacement and, consequently, completing at the top of the bed appears prudent in a vertical well. High producing rate tends to leave higher remaining oil saturation than those do at moderate rates. A combination of high-anisotropy and low- length-to-thickness ratio makes horizontal completions unattractive in the crestal area of the field.
Complex reservoir flow problems could be better understood through a study using physical models. The purpose of this paper is to present results of an experimental study of water crest behavior under horizontal wells. This work is made in an effort to better understand the formation and growth of water crest prior to and after water breakthrough. The physical model constructed differs from others in that variation in length and position of the horizontal well can be made. Eighteen different systems with varied oil column thickness and oil viscosity were run. Particularly in systems with viscous oil, the bottom water never reached the tip end of the well even at a producing water cut of almost 100 percent. The end effects defined as the unswept oil are pronounced as the well length is reduced and as the oil viscosity is increased. Also, at high water cut, the portion of the wellbore with low productivity increases with well length. These physical occurrences have not been previously reported. Postbreakthrough performance will be presented and discussed.
While water production is an inevitable consequence in bottom water reservoirs, it is usually desirable to defer the onset or the rise of water coning as long as possible. Numerous mechanical and chemical methods have been applied to achieve this goal over recent decades. This paper presents new insights into improving oil production and reducing water production by considering flow barriers below horizontal well trajectories in formation regions with low permeabilities, especially natural ones such as shale bodies.
A 3D numerical simulation that applies the Computer Modelling Group's (CMG) STARS Simulator as a cost-effective way to investigate the effects of barriers on horizontal well performance in a bottom water reservoir has been conducted. More specifically, the effects of permeability, dimension, and position of barriers have been comprehensively analyzed when a horizontal well is implemented as a producer. The simulation results have shown that if barriers exist below a horizontal producer, water cut can be postponed and reduced greatly, and cumulative oil production can be increased. Cumulative water production can be decreased dramatically as well. To broaden the applicability of this new insight, some of the possible field implementing technologies, including fractured horizontal wells, small-scale CO2 injection, and solvent injection, are qualitatively simulated to determine their applicability for developing heavy oil in bottom water reservoirs using horizontal wells with the presence of barriers. The simulation results have shown that much better well performance can be reached with the help of barriers when these technologies are integrated systematically.
This new strategy shows a rather promising and economic way to develop bottom water reservoirs where the natural driving energy of the aquifer could benefit the oil production process. The results and understanding acquired from this study offer insights into the development of bottom water reservoirs.
Introduction
Water coning is a critical issue for conventional vertical well production in bottom water reservoirs. Generally speaking, the fluid production process creates a low-pressure region around the wellbore in the reservoir. This differential pressure causes the oilwater interface to deform into a cone shape, at which time the less viscous water phase is produced in preference to the more viscous oil phase. Consequently, the producing Water-Oil Ratio (WOR) increases quickly and readily reaches an uneconomic level. Darcy's law is still the fundamental principle behind this phenomenon. In the past, many researchers have conducted experimental, analytical, and numerical studies on water coning behaviour in vertical wells. Muskat and Wyckoff(1) published one of the early studies related to water coning in 1935. They observed that water coning is a rate-sensitive process and determined the critical oil rate, which is the maximum water-free production rate, by neglecting the shape of the cone. Later on, researchers(2, 3) directed their studies toward the calculation of the critical oil rate. Guo and Lee(4) indicated that the critical rate does not occur at zero wellbore penetration, as may intuitively be expected, but at a wellbore penetration of about one-third of the total oil-zone thickness for an isotropic reservoir.
Stable cones created by a horizontal well drilled with maximum clearance from the unwanted fluid are studied (Muskat9 type approach, using pertinent viscous flow potential), yielding practical values for the critical rate per unit length of a horizontal well, and for the critical cone elevation, depending on both vertical and horizontal permeabilities, oil thickness, both fluid gravities, and oil viscosity. Critical rate weakly decreases when vertical permeability decreases.
The same type of analysis is repeated for a vertical well completed over a very short interval (modelized as a point source) also with maximum clearance from the gas cap or water table, yielding values of critical rate and cone height for the vertical well. In this case the critical rate slightly increases when vertical permeability decreases.
The outcome of a comparison between both types of wells shows that a critical cone comes closer to the horizontal well than to the vertical well. Also horizontal wells generally allow higher critical rates than vertical wells, however this advantage lessens with increased anisotropy. Very simple formulas are provided to evaluate what gain in critical rate may be obtained from a horizontal well.
The paper presents the experimental study that is flow mechanism of bottom water breakthrough into oil well with horizontal well of barrier interbed and permeable interbed was studied, and reservoir physical simulations of barrier layer and permeable interbed flow of horizontal well were performed by 2D physical visible model. The model is made of two polymethylmethacrylucote planes of transparent and the metal frame that dimensional is 700mm300mm14mm.The production mechanism and flow behavior of bottom water reservoirs of barrier interbed and permeable interbed horizontal well was revealed. All development process of water cresting of barrier interbed and permeable interbed were shown by the experiment. Meanwhile, the some rules that are point displacement no better than the displacement of bottom water, and there is no phenomenal of water cresting to the permeable interbed, and result in water-free recovery percent of reserves and terminal oil recovery both are enhancement. The authors discovered that the weak permeable interbed had high water-free recovery percentage of reserves and terminal oil recovery after the weak permeable interbed compares with the barrier interbed. The experiment reveals the to effect horizontal well production rate on main sensitive factors, such as the horizontal well length and the displacement pressure drawdown. The paper presents also same horizontal well length to increase pressure drawdown about two times then terminal oil recovery to decline 3.14per cent and water-free recovery percentage of reserves to decline 13.7per cent to the weak permeable interbed. The bottom water reservoir of weak permeable interbed and barrier interbed both should develop to mitigate water cresting and prolong bottom water breakthrough into oil well in the some degrees respectively.
Using the multilateral well to build up manual barrier layer for the water control in bottom water reservoir with horizontal well. The new concept development has been established through the experiment.
Most authors have concentrated on correlations for critical rate and breakthrough time in vertical and horizontal wells. WOR (water-oil ratio) has also been addressed in vertical wells. However, WOR performance in horizontal wells has not received much attention. The purpose of this work is to develop a method suitable for either hand calculation or simulation to predict (1) critical rate, (2) breakthrough time, and (3) WOR after breakthrough in both vertical and horizontal wells.
An extensive sensitivity analysis of water coning was performed using numerical simulation. From this analysis, an empirical coning correlation was developed based on the basic flow equations and regression analysis. The format of the correlation is similar to Addington’s gas-coning correlation. It predicts critical rate, breakthrough time and WOR after breakthrough.
WOR performance at variable rate production conditions has also been evaluated in this work. It was found that WOR has hysteresis, (i.e., WOR not only depends on the current production rate, but also the previous production history). However, given sufficient time after rate changes, hysteresis disappears. At such conditions, the correlations can also give a good estimation of WOR for variable rate cases.
This correlation provides a hand calculation method of coning prediction for both vertical and horizontal wells. It can also be used as a coning function for 3-D coarse grid reservoir simulation. The correlation was tested and found to be reliable and accurate in predicting WOR, as well as critical rate and breakthrough time when water-oil mobility ratio is smaller than 5 or viscous forces are not dominating.
Gas and/or water coning encountered in many oil wells is a serious problem which results in lower oil production rates, lower oil recovery and increased lifting cost. In the present work, a simple-to-use predictive tool, which is easier than existing approaches, less complicated with fewer calculations, is formulated to arrive at an appropriate prediction of dimensionless breakthrough times as well as optimum horizontal well placement in homogeneous and anisotropic reservoirs as a function of dimensionless rate and density difference ratio. The proposed simple-to-use approach can be of immense practical value for petroleum engineers to have a quick check on estimating the simultaneous water and gas breakthrough time and the optimum location of horizontal well in the presence of both gas cap and aquifer for wide range of conditions without the necessity of any field test trials. In particular, petroleum engineers would find the proposed approach to be user friendly involving transparent calculations with no complex expressions for their applications.Research Highlights► Determination of optimum horizontal well placement in homogeneous and anisotropic reservoirs. ► Calculation of the corresponding breakthrough time for optimum horizontal well placement. ► Development of simple-to-use mathematical predictive tool.
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