A client who is an international oil and gas operator has managed to increase oil production in one of their offshore assets in offshore Malaysia by 15%. Meanwhile, they succeeded in arresting field decline by continuously conducting technical analysis on their integrated production system, identifying opportunities and executing well intervention activities and production optimisation cost-effectively.
This paper is adapted from SPE MS-189208: Value of Portable High-Frequency Multiphase Flow Meter Testing for Brownfield Well Production Optimisation and Maximising Reservoir Performance - A Case Study of An Offshore Brownfield in Peninsular Malaysia (Wee et al., 2017).
Challenges of Aging Oil Field
Ageing oil fields (or brownfields) are challenged by issues such as declining well performance attributed by reservoir pressure depletion, raising watercut and raising GOR (Gas-Oil-Ratio), poor integrity status of the facilities (well integrity and topside facility integrity), bottlenecks within the process facilities (gas compressor performance, subsea pipelines) and other flow assurance issues (sand production, wax deposition. Due to the system's complexity, highly experienced engineers and high technological solutions are required for day-to-day business operation and getting the most out of the ageing fields.
Solving the Problem by Addressing the Fundamentals
Well test data is the fundamental building block of field surveillance and production optimisation. The data can lend itself to be used in a variety of key activities such as production allocation, reservoir performance analysis, well performance analysis, production planning and production forecasting (see figure below).
Framework of Well-Test Supporting Field Surveillance and Production Optimisation
Among the numerous challenges faced, poor well-test facilities (production well-testing) was recognised to be yet another significant barrier to achieving production optimisation. At one point, it was found that 50% of the existing well-test facilities were not fit for operation due to equipment malfunctioning and severe corrosion or wall metal loss. This means the well-test data were not accurate, in some cases misleading. Due to this finding on the data gap, it was generally recognised that effort on production optimisation had reached a limit.
In addition, the field operation has a constraint in produced gas handling imposed by the gas conditioning and compression facility capacity limit. Therefore, having the optimum well mix with optimally low GOR is crucial for maximising oil production. However, this is impossible if the quality of well-test data is poor or inaccurate.
Well test data is critical for field surveillance and maximising reservoir recovery. Both short-term and long-term plans should be made for improving well-test data. The strategy here includes closing the data gap faced by addressing the short-term and the long-term need.
When a new portable MPFM (Multiphase Flowmeter) unit was brought in to perform well-test, it was evident that the benefit was immediate, especially for the platform where the gas rate measurement meter in the existing well-test facility was not working (see figure below). The gas rate measurement from the portable MPFM now allows the engineers to understand the GOR of each well. This finding was critical data for optimising well performance and maximising oil production given the fixed-capacity limit of the gas compressor.
Improvement of Gas Measurement for Wells by Portable MPFM (Visualization in ResWELLTM software tool)
The use of MPFM with high-frequency data recording was beneficial. It made it possible to identify well with an unstable flowing condition (slugging) by just looking at the high-frequency well-test data. With this information, the production engineers can change well parameters such as flowline choke size and gas-lift injection rate and understand if this will improve well performance.
High-Frequency Well-Test Data Record Showing Dynamic Flowing Condition
As more new well test data were gathered, we noticed that the statistics of non-conforming well test data were significantly high in some wells. When we investigated further, we discovered that the FTHP of the well was showing an irregular trend (illustrated in the figure below). We suspected that the downhole subsurface safety valve (SSV) could be partially opened due to the SSV hydraulic line pressure not holding. One possible explanation could be a potential small leak within the wellhead panel hydraulic system (which is not uncommon for wells of such age in this region). Subsequently, wellhead maintenance was performed on the well, and new well test data gathered after that showed the well was producing at a higher production trend. It was a promising finding that not all non-conforming (or bad) well test data are useless. Some can be proven to be quite helpful!
Increase Field Production by 15 – 20%
An on-site optimisation exercise at a platform involved changing wellhead choke opening and shutting in and on different wells has led to understanding which well-mix configuration is optimum for production. The figure below illustrates an example of the findings; when a particular well was shut-in, we could observe from the real-time MPFM well-test data that the observation well began to produce 15% more liquid. Using the observation as an analogy, we could deduce that the dynamic backpressure effect to other wells was also minimised, leading to more liquid production from the wells overall.
Onsite Platform Well Mix Optimisation using MPFM real-time well test data
In a mature oil field, a high activity level of well intervention works for idle wells re-activation and production enhancement. 20 – 30% of the flowing wells are from re-activating idle wells throughout the year (except during monsoon weather period). A new reservoir zone (from add-perforation work) was brought to production among the restored idle wells. In such a situation, the well behaviour from the new reservoir zone was less understood of, and therefore close monitoring of the well performance using MPFM was required. This ensured that well operation was kept within optimum conditions – no premature water or gas production and excessive sand production. During well performance monitoring with MPFM well-test (see figure below), it was observed that the well was slugging (at 1-hour frequency and 100psi pressure heading) at 60% wellhead choke opening. Each time a well slug was observed, a small amount of water was being produced simultaneously. Subsequent well performance analysis with well model (nodal analysis) showed that the estimated drawdown was at the specified drawdown limit, and hence, further opening of wellhead choke was not preferred. Meanwhile, it was suspected that water production during well slugging could be due to high fluid velocity pulling water into the wellbore, similar to water coning. The production team subsequently choked back the well to 50% choke opening (from earlier 60%), leading to a more stable production with almost no slugging and lesser water production.
Re-activated Idle Well Monitoring, Drawdown Control, Slugging, Premature Water and Choke Optimisation
The improvement in well test data will help improve the understanding of well performance and lead to better identification of production optimisation. One example was gas-lift valves change out (GLVC) for gas-lift optimisation in a producing oil well (illustrated by the figure below). Well-test data was initially used to diagnose sub-optimal well lifting, design the new gas-lift valves, and lastly, post job well performance monitoring. A higher oil production (by 150%) with high gas-lift injection (high gas production from well) was recorded after the GLVC. Moreover, the well test data allows the production team to investigate multiple optimisation scenarios for offsetting well oil production decline due to increasing watercut or GOR trends. With the data, the production team can make the correct planning or the most effective and efficient route to optimise the well-mix and choke configuration (assisted by well modelling and IPM – Integrated Production Modelling technique) for better field production performance.
Value of Data: Well Test Data Enables Well Optimisation (Gas-Lift Optimisation)
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