New Approach for In-Line Production Testing for Mature Oil Fields Using Clamp-On SONAR Flow Metering System
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New Approach For In-Line Production Testing for Mature Oil Fields Using Clamp-on SONAR Flow Metering System Giovanni Morra, Sara Scagliotti (Eni E&P) Dhia Naeem, Ali Al Alwan (South Oil Company Iraq) Siddesh Sridhar, Ahmed Hussein (Expro Meters) Abstract Monitoring produced oil, gas and water rates from individual wells plays an important role in reservoir management and production optimization. While beneficial, obtaining timely and accurate wellhead measurements can be challenging due to a range of factors. This paper describes a cost-effective and convenient approach to production surveillance of black oil wells using clamp-on flow meters (Sonar), integrated with a PVT and multiphase flow engine to calculate the properties of the produced fluids, and the individual phase flow rates. The PVT engine calculates the gas and liquid properties of the produced fluids, at the pressure and temperature conditions measured where the Sonar flow meter is clamped-on. The Sonar flow meter provides a direct measurement of the mixture flow velocity within the flow line. The mixture flow velocity is then interpreted in terms of actual gas and liquid flow rates. Once the gas and liquid flow rates are determined at actual conditions, the oil, water, and gas flow rates are reported at standard conditions based on the PVT data calculated by the PVT engine. The new approach was deployed in a mature field to test 6 different wells against existing conventional test separators as the reference. The tests showed that Sonar meters could be installed either upstream or downstream of the choke manifold thus allowing more flexibility in terms of field installation. Some of the wells were tested at fixed flowing conditions while others were tested at multiple choke settings. All data is found to be within acceptable limits. The results show that, overall, the new Sonar-based approach could perform within ±15% or better from the reference, depending on the accuracy of the input PVT data (including water cut) and the flowing conditions. The new clamp-on approach requires about 90 minutes for installation and commissioning which allows the possibility to perform multi-rate testing of the wells in one day. Therefore, the Sonar clamp-on methodology offers the opportunity to increase the well rate measurement frequency at a field-wide level thus allowing a field production back-allocation with consequent benefits in field/production management. Introduction Accurate measurement of net oil rate from individual wells is a critical component in effective oil field management, influencing production optimization strategies and allocation issues. The main equipment currently used for this purpose in the field is either the CTS (Conventional Test Separators) or the MPFM (conventional Multi Phase Flow Meter). The separators in the field DGS (De-Gassing Station), which were originally designed as test separators, are currently being used as Production Separators (PS).The field under investigation is characterized by both carbonate and sandstone reservoirs on production since more than 60 years. A total of 240 wells have been drilled, of which more than 100 (40 in natural flow and 60 completed with ESP) are on production and 40 are water injectors. Assuming that each producing well should be tested at least twice a year, more than 200 well tests need to be performed per year. Considering the fact that each test takes an average of 3 days, the consequences are quite significant: a. Multiple well test packages would be required to accomplish the desired testing frequency, which may not be available and may be cost prohibitive. b. The amount of gas and oil flared to the atmosphere may exceed environmentally accepted levels, unless the existing wellhead connections and well test equipment are modified. c. The production losses/deferral associated with traditional well testing methodologies may be significant. d. Shutting in wells to rig up test equipment can negatively impact the long-term performance of the well. e. Higher HSE risk associated with the increased number of well tests. The solution of re-injecting the produced fluids into the flow line has been already considered for the new well site facilities. To achieve this, it is necessary to have two gate valves in the production flow line, one to connect the well with the CTS and the other one for re-injection. The MPFM offers a flare-less alternative, but it is usable only where the two gate valve system has already been implemented at the well site. In order to address some of the shortcomings described above, it was decided to evaluate the capabilities of Sonar metering as an alternative for well testing. 2 To evaluate the feasibility of using the clamp-on Sonar as an alternative for well testing, it was initially decied to do a preliminary trial on two wells. The tests were carried out on wells A-01 and A-02 in conjunction with ongoing production testing activities. The CTS was used as a reference to check the Sonar meter accuracy. At the conclusion of the preliminary trial, it was deemed that the technology was suitable for the field, providing sufficient accuracy and easy handling in terms of field deployment and installation. It was consequentially decided to deploy Sonar metering on a larger scale in a field-wide well test campaign.The field-wide campaign comprised the testing of 94 wells (including 3 water injection wells) over a period of 108 days. The Sonar results from the individual producing wells were aggregated and compared to the total production rates as measured at the Degassing Stations. The aggregate Sonar production data was in reasonable agreement with the actual production rates as measured at the DGS and to hydraulic well models expectations. In order to address some differences seen between the Sonar test results and the expected flow rates, 4 wells were retested against the CTS at the conclusion of the Sonar field-wide campaign. Again, the results showed that the Sonar meters performed within expectations. This paper presents the results of the Sonar testing, including detailed comparisons between the Sonar and CTS oil, gas and water rates for the six wells where reference data is available. a. Production Allocation / Well Testing Worldwide, the majority of oil & gas production allocation is achieved using separator-based measurements. Test and production separators are generally used to allocate production from individual wells and fields. Most separator-based measurement methodologies rely on the assumption of complete separation of the gas and liquid phases prior to measurement. In practice, however, complete separation is often difficult, if not impractical, to achieve, the accuracy of separator- based measurements could be limited by flow instrumentation-related issues. On the flow measurement side, the test separator functionality may be limited by the quality of the incoming fluid stream, which many times requires further processing, such as de-sanding (or solids removal), in order to protect both the separator and the ancillary instrumentation. In addition, sanding and/or solids may cause the separator to be by-passed for certain periods of time, with no flow measurements available during the by-pass period. Another potential issue is incorrect selection and sizing of flow meters on the liquid and gas legs of the CTS (for example, wrong orifice plate size) which may yield inaccurate measurement results. The size and the cost associated with mobilizing and operating CTS packages in the field may also limit the number of wells to be tested. Therefore, for field management purposes, various schemes and models are often developed to infer the production rates for each well. These models are used to predict individual well performances and in turn to make selection of intervention and work-over methods based on incomplete data with regard to actual well performances. To reduce the uncertainties of the models and the consequent risk associated to the selection of the interventions, it is mandatory to strengthen them with measured data. This data has been provided efficiently by Sonar Clamp-on meter within the required timeframe. b. Overview of Technology Clamp-on Sonar-based flow meters utilize Sonar processing techniques to determine the speed at which naturally occurring, coherent flow structures convect past an array of sensors clamped-on to the outside of the pipe. Figure 1 illustrates the naturally occurring, self-generated, coherent structures present within turbulent pipe flows. Naturally occurring, self-generating, turbulent eddies are superimposed over the time-averaged velocity profiles. These coherent structures contain fluctuations with magnitudes on the order of 10 percent of the mean flow velocity and are carried along with the mean flow. These eddies remain coherent for many pipe diameters and convect at, or near, the volumetrically-averaged flow rate in the pipe. 3 Figure 1 : Sonar-based Flow Meters with Coherent Structures within Pipe Flows In Sonar array processing, the spatial/temporal frequency content of sound fields are often displayed using “K-ω" plots. K-ω plots are presented as surface plots in which the power of a sound field is allocated to bins corresponding to specific spatial wave numbers and temporal frequencies. On a k-ω plot, the power associated with coherent structures convecting along with the flow is distributed along “the convective ridge”. The slope of this ridge indicates the speed of the turbulent eddies. Thus, identifying the slope of the convective ridge determines volumetric flow rate. Slope – 34 ft/sec K Figure 2: K-w plot from a Clamp-on Sonar meter on a Gas Well Figure 2 shows an example of a k-ω plot generated from the diagnostic output of a Sonar flow meter clamped on to a 4-inch, schedule 80 pipe with black oil flowing at 30 barg. As shown, the k-ω plot exhibits a well-defined convective ridge.