Well testing during the drilling of a well using a Drill Stem Test (DST) tool is a well-established practice in the oil and gas industry. As the name suggests the well test is conducted using the drill stem with the DST tool replacing the drill bit. It is done in open or cased hole and the objective is to measure the pressure, permeability and productivity of the zone of interest. It also gives a measure of the damage done to the formation during the drilling process.In the case of an oil reservoir oil is produced to surface during the DST operation and either collected in tanks or flared together with the associated gas. In the case of a gas-only reservoir the gas produced to surface is flared.Some of the issues of carrying out a DST test are the high costs involved, limitations in acquiring detailed zone specific information instead of averaging over the well test length and the potentially negative impact on the environment from hydrocarbon flaring. It is to overcome these challenges that the industry looked for alternative well test approaches that can be more efficiently undertaken during drilling operations such as the Wireline Formation Test (WFT), also known as a mini-DST.A mini-DST well test is carried out using a wireline tool and is conducted in open-hole. The well test involves isolating a zone of interest using packers with pressure and hydrocarbon flow rate readings captured using a single probe, a dual probe or a combination of single and dual probes. Unlike the DST it can be done over multiple zones, isolating one zone at a time and has an added advantage that all fluid flow takes place down-hole.In reservoir intervals with good permeabilities and high production rates, the relatively high mobility of gas compared to oil will result in turbulent flow being observed in the wellbore thereby introducing 'noise' that will mask the sensing of pressure transients by the probe. This is not the case in mini-DST test as the production rates are quite low already. However, for a gas reservoir it is sensitive to the permeability-thickness (kh) product in the zone of interest, since higher kh and lower gas rates can result in a very small drawdown. Therefore pressure gauge sensitivity becomes a significant factor in the well test design for a mini-DST. With a typical gauge resolution of ca. 0.01 units there is, therefore, an inherent constraint to targeting primarily low kh flow zones. This can result in a trade-off when selecting reservoir units to test based on their permeability (k) and thickness (h).This paper looks at some of the issues to be considered in the design and operation of a mini-DST well test for a gas reservoir and highlights key benefits that can be derived. It further discusses some of the operational challenges encountered during a case study.
Hydrocarbon exploration is a challenging, costly and high-risk investment and success often requires further drilling of one or more appraisal wells to acquire data of the highest quality for reservoir evaluation and characterization before development plans can be put in place. During the exploration and appraisal stages it is important to define the reservoir fluid properties and reservoir parameters to enable accurate economic assessments. Three appraisal wells in two gas fields offshore Trinidad and Tobago were drilled in reservoir formations that are mainly unconsolidated sandstones with dry, biogenic gas. An extensive wireline logging program was planned which ranged from basic and advanced petrophysical logs to images and formation testers including the dual-packer module to perform interval pressure transient tests (IPTT) or ‘Mini-DSTs’ as these are also known. This was the first time in Trinidad and Tobago that IPTTs were conducted on appraisal gas wells. Technical factors influencing the decision to use an IPTT as opposed to a full Drill Stem Test (DST) included access to data from existing full DST's acquired in a nearby offset wells and a specific need to target thin-bedded and laminated sands so as to evaluate them individually and to observe vertical interference or connectivity between sand units. A combination of IPTT analysis and real-time downhole fluid characterization was applied to the appraisal wells which resulted in an improved understanding of the reservoir. The IPTT planning, methodology and applications, along with insights on some of the challenges encountered during operations which included changes to drilling operations in order to achieve the planned formation evaluation are presented.
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