SOCIETY
OF PETROLEUM ENGINEERS
Mid-Continent Section, Tulsa, Oklahoma
Abstracts courtesy of Ken Saveth, ksaveth@juno.com
"Analysis and Optimization of Progressing Cavity Pump Systems by Total Well Management" - Jim McCoy: Echometer Company
The need to increase oil production, reduce operating costs and increase net income from PC pumped wells, requires an integrated analysis of the pumping system including the performance and interaction of al the elements: the reservoir, the wellbore, the downhole pump, the rod string, the drive head, and the prime mover. Such system analysis can now be undertaken efficiently using portable laptop-based data acquisition systems in conjunction with the appropriate transducers and a suite o analysis software.
Field experience has resulted in the development of a procedure which insures that good results are obtained with the minimum of effort. It is the objective of this paper to outline this procedure or methodology with the hope that it will be of use to production personnel throughout the industry. The end result of such system analysis should be the complete visualization of the performance of a given pumping well at a given time and a set of recommendations to be followed if significant improvements can be achieved.
In general, the following steps should be taken:
1. Establish the well’s inflow performance to determine if additional production is
available
2. Determine the overall system efficiency as a means to identify wells which are inefficient.
& thus are candidates for mechanical/electrical changes to improve performance.
3. Analyze performance of downhole pump & downhole gas separator.
4. Analyze mechanical loading of rods.
5. Analyze performance of prime mover.
6. Design possible modifications to improve existing system.
7. Implement cost effective changes & verify improvement.
METHODOLOGY
The steps to be followed in defining the performance of the system should result in the maximum of information with the minimum of time and effort. This is facilitated by having access to accurate and timely information about the well’s characteristics, completion, well tests, etc. Although this may be initially time consuming, it is generally a task that has to be completed only once. Some sort of data base management system is helpful in maintaining this information current. The information should be organized by well into a summary Well Data File which can be accessed by most of the application programs that are likely to be used. This eliminates having to re-enter the data.
BASIC SURVEY
Certain basic measurements are necessary to identify those wells that are the most likely candidates for improvement. experience has shown that this can be accomplished cost effectively by establishing the well’s inflow performance (by measurement of the liquid level depth and casing pressure) and measuring the overall efficiency of the pumping system. This requires only the measurement of input power to the prime mover, determination of the producing BHP and accurate well test data. The producing BHP is obtained from liquid level and casing pressure measurements and then computed taking into account the affect of annular gaseous liquid column if present. If a digital acoustic fluid level survey is not available, the calculation of the PBHP from strip chart liquid level depth and casing pressure measurement can be performed on any PC using the program IPA or AWP which can be obtained free of charge from Echometer Company. Measurements of the fluid level and casing pressures also allow the determination of the net lift of the liquids.
The basic survey will indicate whether the efficiency of the pumping system is adequate and whether a low PBHP is present in the well. If either of these conditions is not satisfied, there is an indication that modification of the system or operating parameters could be beneficial and allow improved operation.
Without including the compilation of well data, and acquisition and processing of the acoustic, pressure and motor power/current data can be accomplished in about 45 minutes per well. This rapid basic analysis should be efficient to properly analyze the performance of the system in more than 90% of the wells which need to be tested.
Inflow Performance
Additional surveys may be required to characterize the inflow performance of the well. This seems to be one of the most deficient areas in most fields.
Power Analysis
One objective of acquiring power data is to determine the efficiency with which the pumping system is being operated from the standpoint of energy utilization & mechanical loading. One thing in particular that can be studied is the torque on the rods.
Gas Interference Optimization
The PC pump intake must me filled with gas-free liquid for high volumetric efficiency. For this reason, the pump should be set below the formation or a proper downhole gas separator should be run below the pump to minimize free gas entry into the pump. There are several drawings of downhole gas separators contained in the paper; however none are as good as setting the pump below the formation.
An efficient PC pump system requires liquid without free gas bubbles at the pump inlet. A table contained in the report; Adiabatic Compression of 0.8 SG Hydrocarbon Gas; shows the temperature of gas when it is compressed from the inlet conditions, to the pressure which is at the discharge of the pump. There can be a tremendous increase in temperature due to this compressing of the gas. Since high temperatures can damage the PC pump, this occurrence needs to be carefully looked at when high percentages of free gas are present. Probably, when free gas is present, pump inlet pressures in the mid-range of 5 to 500 PSIA will result in the most heat being transferred from the gas to the PC pump components thus reducing pump life. During normal producing conditions, the inlet & outlet pump conditions generally remain relatively constant. If liquid is entering the pump at 50 PSIA & 100 ºF, the liquid is discharged in approximately 3 seconds into the tubing above the pump with very little change in temperature. The discharge pressure in a 3,000 ft. well will be approximately 1500 PSI. On the other hand, assume that a slug of gas exists at the pump inlet & only gas is available. The gas at 50 PSIA & 100ºF will be compressed in approximately 3 seconds to an outlet pressure of approximately 1500 PSIA. If this is an adiabatic compression, the temperature would approach 490ºF near the pump discharge within a few seconds if liquid is not available to cool the compression process. It is obvious then, to see why it is important to minimize the amount of free gas that is seen at the pump intake.
TOTAL WELL MANAGEMENT COMMITMENT
This approach to well management requires a commitment on the parts of operating personnel to the concept of the well as a system of interrelated elements, each of which can have a major effect in changing the efficiency of the system. This concept may be foreign to some operators for which a "well" comprises the downhole pump only, or the rods, or drive unit, or formation flow characteristics depending on their past experience & responsibility. Therefore, successful application of the Total Well Management concept generally includes some training of the operating personnel. This is especially necessary if effective application of modern data acquisition hardware & software is to be introduced.
SUMMARY
Operation of PC pumped wells using the concept of Total Well Management results in a more complete understanding of the performance of a given well. Implementation of this concept can result in significant increases in oil production & reduction in operating cost.
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