Expansion of oilwell cement after placement in the annulus is a promising route to improve wellbore sealing. Expansion can potentially provide benefits such as: closing a microannulus, reducing the tendency for cracking/debonding, and improving cement logging evaluation. To properly evaluate the performance of expanding cement formulations in a laboratory setting, the cement should be hydrated under confined conditions similar to a wellbore. We conducted measurements using a new confinement cell developed in our laboratory. The cell provides radial confinement and elevated temperature as in a wellbore. The availability of water to the cement can also be controlled to simulate tight or porous formations. In the axial direction the cement sample contacts a piston that either confines the sample while measuring the axial stress development, or permits linear expansion that is measured. Using blends of oilwell cement with magnesium oxide (MgO), a standard expanding additive, we measured the axial stress development and linear expansion under different curing conditions. To better understand and optimize cement expansion we also characterized the hydration kinetics of the MgO additive alone and in the presence of the cement, using isothermal calorimetry. Finally, we measured the acoustic response of cement formulations with and without a commercial expanding agent with a standard ultrasonic pulseecho technique. This testing confirmed the benefits of expansion for the logging response.
Depth correlation before perforation is crucial in reservoir testing, to minimize the uncertainties about the reservoir being tested. Currently, three main methods are used to correlate to depth: simple pipe tally, tag of a known element in the well, or a wireline gamma ray (GR) log—which is the most accepted method, owing to its precision and track record. Wireline GR correlation, however, requires additional time (6 to 8 hours), translating into cost for the operator. We describe a new depth correlation tool, which is mounted directly in the drillstem test (DST) string and is enabled by a downhole wireless acoustic telemetry system, that provides correlation algorithm in real time without the need for extra wireline intervention. The introduction of a wireless-enabled GR measurement into the DST string radically changes the workflow for DST operations and results in significant reduction of rig time, as shown by one of our latest job campaigns in Qatar.
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