CO2-Induced Changes in Oilwell Cements Under Downhole Conditions: First Experimental Results

Abstract

One of the major technological issues for CO 2 injection (for EOR, CCS, etc.) is the long-term behavior of cement-based materials used to ensure the overall sealing performance of the storage wells. When water is present, the CO 2 after injection can react chemically with the cement (i.e. carbonation). How do the CO 2 -enriched formation fluids changes the cement's chemistry and properties? Could the sealing efficiency of the wells be affected by these changes?The objectives of our experimental program are to assess the kinetics and phenomenology of the changes that occur in different class-G Portland cements exposed to CO 2 -enriched aqueous fluids at 8 MPa and two different temperatures.The experimental program presented in this paper consists of: a first carbonation test (Test # 1) using neat G cement, at a temperature of 90°C (194° F) and a pressure (supercritical CO 2 above water) of 8 MPa (1160 psi); a second carbonation test (Test # 2) using G cement with silica flour (to prevent strength retrogression), at a temperature of 140°C (284°F) and at the same CO 2 pressure of 8Mpa. Finally, coupled chemo-mechanical tests (dynamic tests) are underway on similar class-G cement and similar CO 2 -rich water.All the samples were prepared according to ISO/API specifications. The experimental set-up simulates downhole "static" conditions: the samples were immersed in water in a cell thermally regulated and pressurized by CO 2 . Cement samples were exposed to CO 2 -saturated water for various lengths of time (from one week to 3 months) and were characterized using advanced methods for chemical and mineralogical analysis (X-ray tomography, SEM, XRD, TGA-TDA…) and mechanical testing.The main preliminary results show a reactive front (characterized by carbonation) progressing from the fluid-sample interface towards the sample centre. The carbonation front moves faster during Test 2 (at higher temperature) than during Test 1 (at lower temperature). SEM images of Test 2 also show a thin layer of dissolved carbonate at the sample's surface. The carbonated cement areas exhibit increased density and greater compressive strength.The results of coupled chemo-mechanical tests with injection of CO 2 -enriched water in samples under deviatoric stress show that the CO 2 flow rate in the cement rapidly decreases, finally resulting in carbonation clogging of the cement sample. These results seem consistent with reported field observations.

A review on cement degradation under CO 2 -rich environment of sequestration projects

“…To gather information about the reaction kinetics and phenomenological changes in the cement, Garnier et al, (2010) reported an experiment with two sets of samples. One was neat Class G cement tested at a temperature of 90⁰C and a pressure of 8 MPa for a period of 90 days whilst the second sample was composited of Class G cement containing silica flour (35% BWOC).…”

A pozzolanic supplementary material to reinforce class G cement used for drilling and completion operations

“…This standard was chosen as it is commonly used in the industry for the preparation of primary and secondary cement. Different researchers (e.g., Barlet-Gouedard et al, 2006Kutchko et al, 2007Kutchko et al, , 2008Garnier et al, 2010;Brandl et al, 2010;Lesti et al, 2012;Zhang et al, 2014) also used the API standard for the preparation and testing of the oil well cement. The detailed instruction of the cement preparation and testing procedures used in this paper are given in the API 10-A and 10-B.…”