Over the past few years, there has been a surge of interest in using Australia's coalbed methane (CBM) resources to produce liquefied natural gas (LNG) for export, taking advantage of increasing prices and growing global demand for gas. Principally in Queensland, the second largest state, situated in the northeast of the country, there has been significant investment by a number of companies to secure footholds in this resource play. By December 2008, seven proposals for LNG plants had been announced, most involving partnerships between Queensland companies with coal seam gas resources and international petroleum companies. Most exploration and appraisal CBM wells are tested to determine pressures and coal seam deliverability. As the majority of wells cannot flow to surface naturally, closed chamber tests are conducted, most often in open hole. An alternative testing method employed by a number of operators in Australia over the past 2 years has been the dual packer module of wireline formation testers. Although wireline packers provide a cost-effective and an efficient way of obtaining high-quality pressure transient data sets, they are limited to relatively small test intervals. A new, innovative approach enables testing coal seams of up to 15 m in thickness using wireline formation testers. The new wireline formation testing configuration offers several benefits over conventional drillstem tests (DST) for this application, including rig-time savings. Another benefit of the new technique is the broad range of drawdown and inflow rates, extending coverage from very low- to high-permeability coals. Additional applications used in the field include step-rate tests, the results of which have been applied to assess and rank different coal seams across a number of fields and coal types.
Following the first commercial recovery of coal-seam gas (CSG) in the 1990s, the CSG industry in Queensland has grown rapidly due to the abundant reserves (~33 000 PJ) and global demand for LNG. There are currently three LNG export projects under construction on Curtis Island near Gladstone in Queensland, with the first shipment scheduled for middle to late 2014. In preparation for the completion of the first LNG plant, the QCLNG project, operated by QGC, is currently ramping up CSG production in the Surat basin. By the end of 2014, more than 2000 wells will be drilled and connected to the QGC gas-and-water-gathering network, and thousands more are scheduled for drilling and completion in subsequent years of the project. To meet the production target, it is increasingly important for QGC to accurately evaluate the well productivity during the early stage of development to optimise the operation of the field and ensure all LNG contractual agreements are met. To date, drillstem testing (DST) has been the most common form of evaluating CSG well productivity. However, the improved capabilities of the wireline formation tester (wireline FT) for testing larger coal intervals (between 1 and 15 m) and deriving similar reservoir parameters is making it a popular alternative to the more costly DST. Nevertheless, although facing very few issues in low-permeability coal seams, the wireline FT is unable to create sufficient pressure drawdown in higher permeability coal seams (>100 md) due to the mechanical limit of its downhole pump (<20 B/D). As a result, the wireline FT interval pressure builds up to formation pressure rapidly and stabilises to gauge resolution within the first few minutes. This causes significant uncertainty in the permeability measurement. In view of these wireline FT limitations, operators have been more inclined to evaluate the higher-permeability coal intervals with DST services, therefore motivating wireline service providers to find ways of improving wireline FT capability This paper describes utilising super flow technique to create sufficient drawdown in highly permeable coals using wireline FT sample chamber module. Superflow simulates closed-chamber DST by using the high volume sample chamber module of the wireline FT tool. Two different superflow techniques have been implemented with positive results. Several superflow tests were compared with conventional drawdown/build-up tests over a number of coal intervals of varying permeability to confirm the validity of the new testing method. The results indicate this technique has extended the wireline FT capabilities to higher permeability coals that have been typically conducted by DST services. The addition of the sample chamber module and corresponding superflow theory, the wireline FT has broadened the range of CSG applications.
Well clean-up operation involves the removal of drilling and completion fluids from the wellbore before diverting the well to production facilities. Natural flow clean-up is preferred due to its relatively low cost and simplicity. Depending on the weight of the initial contents in the wellbore and the reservoir properties, artificial lift assisted clean-up such as nitrogen injection through coiled tubing may be required for some wells to ensure the well clean-up objectives are achieved. Well clean-up is transient in nature thus necessitating the need for a dynamic simulation approach to assess the effectiveness of different clean-up options and arrive at the optimal procedure before embarking on the actual field operation. In the current study, a comprehensive-multiphase-transient-simulator (OLGA) was used to predict the clean-up of a gas well with relatively short horizontal open-hole section and low reservoir pressure. Dynamic simulations of clean-up operations for different scenarios such as mud cake lift-off pressures and uncertainties in well productivity were conducted to assess the effectiveness of natural clean-up. Well clean-up failure could lead to impaired deliverability and potential for preferential flow hotspots. The study also assessed if coiled tubing-assisted operations would be beneficial in cases of natural clean-up being ineffective. This paper demonstrates the importance of using transient simulations to provide useful insights into flow and pressure dynamics inside the wellbore during clean-up which can help engineers to predict, design and optimise well clean-up operations, thus increasing the probability of a successful clean-up operation.
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