Effect of Gas-Oil-Ratio on Oil Production
Objectives of the Study
- To examine the effect of Low and High GOR on tubing
- To identify the critical points beyond which production begins to
- To observe the effect of water production at HGOR condition on production
CHAPTER TWO
LITERATURE REVIEW
Overview of System Analysis Approach
Systems analysis, which has been applied to many types of systems of interacting components, consists of selecting a point or node within the producing system (well and surface facilities). Equations for the relationship between flow rate and pressure drop are then developed for the well components both upstream of the node (inflow) and downstream (outflow). The flow rate and pressure at the node can be calculated since flow into the node equals flow out of the node and only one pressure can exist at the node. Furthermore, at any time, the pressures at the end points of the system (separator and reservoir pressure) are both fixed. Thus:
PR – (Pressure loss upstream components) = P node (2.1)
Psep + (Pressure loss downstream components) = P node (2.2)
Typical results of such an analysis are shown in Figure 2.1 where the pressure-rate relationship has been plotted for both the inflow (Equation 2.1) and outflow (Equation 2.2) at the node. The intersection of these two lines is the (normally unique) operating point. This defines the pressure and rate at the node. This approach forms the basis of all hand and computerized flow calculation procedures. It is frequently referred to as Nodal analysis.
Applications of System Analysis Approach
The use of systems analysis to design a hydrocarbon production system was first suggested by Gilbert (1954). Gilbert performed a sensitivity analysis to determine an approximate solution for natural flow and gas-lift problems for 1.9-in, 2 3/8-in, 2.785-in and 3 1/2-in API tubing sizes and crude oils with API gravity ranging from 25 to 40 API. He also explains the hydraulics of natural flow as well as summarizing the methods for estimating individual well capabilities using the same set of API tubing sizes. The author has prepared a very useful tool for the solution of problems relative to flow of oil, gas, and water in a tubing string.
Brown and Lea run a production optimization using a computerized well model. This computerized well model has contributed to improving completion techniques, for better efficiency and higher production with many wells. The optimization was carried out for gravel packed well as well as perforated wells.
Tubings were evaluated for a well to gravel packed. The IPR curve was prepared using Darcy’s law including the additional turbulence pressure drops. The Gulf Coast well was considered for this experiment. Tubing sizes of 2 7/8-in 3 1/2-in and 4 1/2-in are evaluated at the wellhead pressure of 1200 psi needed to flow gas into the sales lines. From the analysis 4 ½-in tubing is selected.
For the perforated well a sample oil well with low GOR, a low bubblepoint pressure, and assumed single-phase liquid flow across the completion was analyzed. The reason for this selection is that current technology has offered solutions only for single-phase flow across such completions. When two-phase flow occurs across a gravel-packed or a standard perforated well, relative permeability effects must be considered. The IPR curve was prepared with Darcy’s law, assuming no pressure drop across the completion. Tubing performance curve was plotted for 2 3/8-in, 2 7/8-in and 3 1/2-in- -in tubing. Assuming 3 1/2-in-in tubing is selected, transfer it pressure drop curve. Using the appropriated equations from Mcleod and as discussed by Brown et al., the pressure drops across the available completions were determined. A final plot is constructed to show the importance of perforating underbalanced.
Rafiqul Awal et al develop a new nodal analysis technique which helps improve well completion of matured oil field. They proposed the use of a simple, tapered tubing string completion (using larger internal diameter ID tubing pipes in the upper sections) that can be customized for specific reservoirs. They employed nodal analysis technique to develop an equivalent tubing diameter (ETD) concept. The ETD allow for comparing the well performance for single – ID tubing. The procedure also seeks an optimum length for the larger tubing ID in the upper section. This method had limitations as it was suitable for wells with moderate to high open flow potential. It is suited for low GOR wells with high future water-cut. The technique was to reduce or eliminate the high capital cost of investing into waterflooding or any other Enhanced oil recovery method.
CHAPTER THREE
METHODOLOGY
Planning the development of a reservoir with respect to sizing equipment and planning for artificial lift as well as evaluating the project form an economics point of view, requires the ability to predict reservoir performance in the future. The reservoir PVT data must be available in order to predict the primary recovery performance of a depletion-drive reservoir in terms of Np and Gp. These data are: initial oil-in-place, hydrocarbon PVT, initial fluid saturation, and relative permeability. All the methods used to predict the future performance of a reservoir are based on combining the appropriate material balance equation (MBE), with instantaneous GOR using a proper saturation equation. However the prediction is narrowed only to the instantaneous GOR. The calculations are repeated at a series of assumed reservoir pressure drops. There are several techniques that were specifically developed to predict the performance of the solution gas drive reservoir. These methods include Tarner’s method, Tracy’s method and Muskat method,
CHAPTER FOUR
RESULTS AND DISCUSSION
In this chapter the nodal analysis described in chapter two is applied for various instantaneous GORs and water cuts. The results obtained from the sensitivity analysis are presented in this chapter.
Results
Tables 4.4a to 4.4e show the variation in oil production rate as well as pressure for a particular range of GOR. The red coloured numbers in Tables 4.3a to 4.3f signify an increase in either the production rate or bottomhole flowing pressure. The blue coloured numbers in Tables 4.3a to 4.3f signify a decrease in either the production rate or bottomhole flowing pressure. Tables 4.3a to 4.3f show the operating points of the various tubing sizes at specific reservoir pressure and .
CHAPTER FIVE
CONCLUSION AND RECOMMENDATION
The results for the oil production trends indicate that GOR production adds some value to production. It is obvious that the GOR produced as the reservoir pressure declines cannot be controlled and as such it is imperative to run a sensitivity analysis for all the available tubing sizes ascertain which tubing sizes gives an appreciable potential gain in production. Tracy’s method was selected as the best among the three produced GOR prediction methods. The result from this study will also serve as a guide in choosing the optimum tubing size during completion stages or at a point in the life of the well and in the gas lift planning.
In HGOR case, potential gain in oil production is hindered and as such the necessary measures can be adopted to remedy or mitigate the situation.
The results from this sensitivity analysis showed that there is always a critical point beyond which there is a net decrease in the oil production rate for all the tubing sizes. The critical point is a function of the tubing size used as well as the current well condition (GOR).
Finally the production of water at any stage of the well irrespective of the percentage also contributes adversely to reducing the oil production rate. All things being equal, the percentage reduction in production reduces as GOR increases from 2610 to 5635 scf/stb for all the tubing size used.
RECOMMENDATION
Due to the limitations of Beggs and Brill, new nodal analysis software such as PROSPER, Wellflo and Snap can be used to carry out this sensitivity analysis with higher level of accuracy.
REFERENCES
- Ahmed T., Reservoir Engineering Handbook, Third Edition, 2006.
- Awal M.R. et al., A New Nodal Analysis Technique Helps Improve Well Completion and Economic Performance of Matured Oil Fields, 2009.
- Beggs, D.H., Production Optimization using Nodal Analysis, Second Edition, OGCI and Petroskills Publications Tulsa, Oklahoma, pp.92 – 95, 2003.
- BOYUN G., et al., Petroleum Production Engineering, A Computer–Assisted Approach, 2007.
- Brown K.E. et al., Production Optimization of Oil and Gas Wells by Nodal Systems Analysis, Technology of Artificial Lifts methods, Pennwell Publishing Co, Tulsa, 1984.
- Brown K.E. and Lea J.F., Nodal Analysis of Oil and Gas Wells, 1985.
- Clegg, D. J., Petroleum Engineering Handbook, Volume IV Production Engineering Operation, 2007.
- ENI S.p.A., Agip Division, Completion design manual, 1999. Gilbert W.E., Flowing and Gas-Lift well performance, 1954. Heriot Watt University, Production Technology 1 lecture notes.
- Lea J.F. and Brown K.E., Production Optimization using a computerized well model, 1986.
- Mcleod, H. O., The Effect of Perforating conditions on well Performance. J. Pet. Tech., January, 1983.
