(79913)_Understanding_Formation_(In)Stability_During_Cementi(4)

钻井固井

4 SPE/IADC 79913

emulsion systems). The membrane efficiency of shale/fluid

systems is due to a difference in mobility of water and solutes (ions) in shales. For shales, when the mobility of solutes is lower than that of water, the membrane is “non-ideal” or “leaky.” The important point is that a nonideal membrane does not entirely restrict the transport of solutes.

If the water activity of the wellbore fluid is lower than the formation activity, an osmotic outflow of pore fluid from the formation, caused by the chemical potential mechanism, may reduce the rise in pore pressure caused by wellbore fluid pressure penetration. If the osmotic outflow is greater than the inflow caused by wellbore fluid pressure penetration, there will be a net flow of water out of the formation into the wellbore. This can result in the lowering of the pore fluid pressure below the in-situ value. The associated increase in the effective wellbore fluid support can lead to an improvement in the stability of the wellbore.

One of the essential parameters that can be adjusted to increase the osmotic outflow is membrane efficiency. The osmotic outflow increases as membrane efficiency improves. In most conventional water-based fluids, the membrane efficiency is low.3,8 Therefore, even if the water activity of the drilling and cementing fluids is maintained significantly lower (with a high salt concentration) than the shale water activity, the osmotic outflow may be negligible because of the low membrane efficiency. The main objective of this project was to identify and understand the interactions between water-based cementing fluids and shale formations. This information should facilitate the design of cementing fluids to help reduce the risk of formation instability during the cementing process. Pressure transmission/chemical potential tests were performed on shale samples to evaluate various compounds for their membrane generation capacity. Details of the experiment and examples of the results are presented in the following section.

Screening Tests for Membrane Efficiency

The screening tests were designed to study time-dependent alterations in shale properties as a result of exposure to drilling and cementing fluids. The experiment consists of confining well-preserved shale samples under geo-static stress and then circulating (under confined dynamic conditions) test fluids at the upstream side of the sample. Change in the downstream pressure is measured simultaneously. The upstream pressure may be increased to simulate overbalance conditions. The application of very high pressure simulates downhole conditions where overbalance is maintained to provide the formation with an effective hydrostatic support. The downstream pressure changes indicate changes in the sample pore pressure. Membrane efficiency is given by the ratio of the maximum differential pressure across the sample and the theoretical osmotic pressure for an ideal semipermeable membrane of the test solution/shale system.

Experimental Setup

A schematic of the membrane efficiency screening test cell is provided in Fig. 3. The screening equipment has six test cells with a confining pressure and pore pressure capacity of 35 and 20 Mpa respectively. Each test cell has an associated test solution cylinder that enables up to six different solutions to be tested simultaneously. This configuration also provides independent control of each test’s start and termination. Two pore fluid cylinders are provided to avoid interruption to the tests when the fluid runs out after switching from one cylinder to the other. Separate high-pressure gas cylinders provide the upstream pressure (test solution) and downstream pressure (pore fluid) that are controlled by high-pressure regulators. The confining pressure is applied with a pump incorporating an accumulator. The upstream pressure of the six test cells is monitored by a single-pressure transducer, while the downstream pressure of each cell is monitored by a separate pressure transducer. Circulation of the test solution is adjusted with the dial gauge of a metering valve at the upstream end of each cell. The entire membrane efficiency-screening equipment is placed in a constant temperature facility to control and maintain constant test temperature.

Summary of Test Procedure

Shale samples of nominally 25 mm diameter and 10 mm long were used in the screening tests presented here. Tables 2 and 3 provide the physical properties and pore fluid composition of the test shale, respectively.10 The experimental procedures for the tests are as follows:

a. Backpressure Saturation. Apply a confining pressure of

20 MPa and a backpressure of 10 MPa with simulated pore fluid at the upstream end of the sample. The downstream pressure increases to above 10 MPa during this stage.

b. Consolidation. The excess fluid/pressure is allowed to

drain/dissipate and the sample is assumed to be essentially consolidated when the change in the downstream pressure is less than 50 kPa/hour.

c. Pore Fluid Pressure Transmission. Upon consolidation

of the sample, increase the upstream pressure to 15 MPa. When the downstream pressure increases by more than 50%, reduce the upstream pressure to 10 MPa.

d. Reconsolidation. Allow the excess pore pressure inside

the sample to dissipate.

e. Test Solution Pressure Transmission. Following the

equilibration of the downstream pressure with the upstream pressure (or stabilization of the downstream pressure), displace the simulated pore fluid at the upstream end with the test solution. Ensure that the volume of test solution pumped is at least twice the volume of pore fluid in the line and upstream platen. Increase the upstream pressure to 15 MPa. Allow the downstream pressure to increase and stabilize.

f. Displacement of Test Solution with Lower Activity

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