1. Field of the Invention
The invention generally relates to a retrievable downhole tool for well testing and a method for testing a well using such.
2. Background Art
Well testing is a common technique used to obtain parameters describing the reservoir and to determine the well productivity. Well testing may be performed at any stage of the lifecycle of a well.
For example, well testing may be performed after drilling the well and before the well is completed for production. Data obtained from downhole instrumentation and fluid samples from a hydrocarbon reservoir provide information such as behavior of the reservoir fluids, formation permeability, skin factors, well productivity, connected volume, pressure, and temperature.
Well testing is also performed to monitor the performance of a production well. The formation pressure is measured by way of repeated pressure drawdown and buildup tests. A mechanically conveyed downhole shut-in valve may be used to shut-in and reopen the well. At the same time, the formation pressure is measured by placing a measuring sensor (e.g., a pressure recording gauge) downhole below the shut-in valve and near the producing formation, i.e., near the reservoir. A pressure drawdown test is conducted by flowing the well, and the well is shut-in for a pressure buildup test.
Typically, there are three well testing methods used in a production or completed well:    (a) performing a flow rate-test at various rates, whereby the well is choked at the well head;    (b) shutting in the well at the well head to conduct a pressure build-up test; and    (c) running and temporarily installing a downhole shut-in tool in the well and fixing the shut-in tool in a landing nipple in order to perform a pressure build-up test.
In either case, the technique of slickline conveyed well testing tools may be used. It consists in lowering a specialized testing tool into the well to a zone of interest (i.e., near the reservoir) using slickline (i.e., a mechanical wire) and reading sensor data from the tool on the fly or stored in the gauge memory. Formation testing tools for slickline testing may also be adapted to obtain fluid samples from the formation. Data collected downhole during well testing may be communicated electronically to the surface for logging. This permits data to be analyzed in real-time.
In all cases (a), (b), and (c), it is assumed to record the downhole pressure close to the sandface, i.e., close to the reservoir, by permanently installed or slickline conveyed pressure gauges. In the case of surface choking and shut-in [(a) and (b) above], large well bore storage or fluid compressibility effects may occur, which mask the reservoir response and increase time needed for stabilisation. Thus, the time required for the test is increased, and it may be impossible to obtain meaningful data about the reservoir. Other well bore dynamic effects as, for example, fluid segregation, may have an impact on both flow rate and flowing pressure stability. Liquid fall back and changing liquid levels may corrupt shut-in data. Furthermore, back allocation of surface flow rates is not always proportional for high gas-oil-ratio wells if the flow rate is controlled from the well head.
In the case of downhole shut-in [(c) above], there are numerous practical limitations, such as the availability of completion nipples to set and seal the tool, the condition of those nipples and thus the potential for leakage, problems with retrieval or re-start of the well, etc. Also, in comparison to drawdown testing in isolation, there is the cost of shut-in and deferred production.
Specifically, there are no cost effective, low risk, and simple methods available to date to assess inflow performance, to determine production potential, and/or to update the reservoir description of producing gas wells. The same applies, to a lesser extent, to oil wells. More importantly in the oil domain, given the large and increasing number of wells with reduced reservoir pressure, there is a risk of killing the well by shutting it in. Thus, a two-fold cost increase is generated due to deferred production and subsequent intervention to recommence production.
Therefore, due to the respective disadvantages of these methods, it is not always possible to obtain interpretable data, and the test objectives may not be met.