Sanding phenomena vulnerability observations due to CO2 injection at the Air Benakat reservoir in South Sumatera

Carbon sequestration using carbon capture storage (CCS) is one of the most important operational activities in the oil and gas industry to reduce greenhouse gas emissions and increase hydrocarbon production. Carbon Capture Utilization & Storage (CCUS) can be utilized for both enhancing oil recovery (CO2-EOR), as well as gas recovery (CO2-EGR) by injecting CO2 into the reservoir. However, the CO2 injection into reservoir rock can raise potential sanding problems in both injection and production wells. The phenomena of sanding may be induced dominantly due to carbonic acid injection in the reservoir. The reaction between CO2 and water will form carbonic acid in the reservoir, which can cause the dissolution of rock minerals, especially carbonate cementation (such as calcite and dolomite). We have investigated the sanding effect when CO2 was injected into the reservoir using several laboratory-scale measurements and observations on the Air Benakat reservoir sample in South Sumatera. The sanding vulnerability was measured by observing pore structure, changes in elastic properties, and rock strength through Rock Physics and Rock Mechanics measurements. XRD analysis showed the presence of CaMg(CO3)2 (dolomite) minerals in the Air Benakat sandstone sample, which resulted in the possibility of a chemical reaction of the sample, either matrix or pores. The pore structure dissolution was detected from microscale images when the rock was injected with CO2 dissolved in brine water. The changes in rock’s pores due to the dissolution process were also clearly observed from measurements of the changes in rock mass during the injection process of carbonic acid fluid into rock samples from time-to-time measurement.


Introduction
Sand production is one of the problems that is often experienced in the oil and gas industry, both in new and old wells.Predicting sand production is key in designing the completion of a well, both production and injection wells.This research focuses on predicting the occurrence of sand problems in production wells in the case of CO2 injection, where CO2 injection activities are currently widely carried out (IPCC, 2005), including for Enhanced Oil Recovery (CO2-EOR), Carbon Capture Storage (CCS), Carbon Capture activities.Utilization & Storage (CCUS) and Enhanced Gas Recovery (CO2-EGR).In Indonesia, Ariadji (2020) said that studies on CCUS had been carried out several times, including EGR studies in the Gundih Block, EOR in the Sukowati and Limau Niur Fields (Pertamina EP) and EGR in the Tangguh Block.
When CO2 dissolves in water, it produces carbonic acid (Greenwood and Earnshaw, 1997, Mitchell et al. 2010, Lerman and Mackenzie, 2018), which is a weak acid with a pH of around 4-5.Some researchers have resulted in CO2 dissolution reaction in formation water (Mitchell et al, 2010) The rate of dissolution of minerals (including calcium and magnesium) is strongly influenced by the pH value (Casey and Sposito, 1992, Matter et al. 2007, Black et al., 2015).The rate of mineral dissolution is also influenced by temperature (Casey and Sposito, 1992) In this study, the investigation of the sanding effect when CO2 was injected into the reservoir has been conducted using several laboratory-scale measurements and observations on the Air Benakat reservoir sample in South Sumatera.The changes in rock pores due to the dissolution process were also clearly observed from measurements of the changes in rock mass during the injection process of carbonic acid fluid into rock samples from time-to-time measurement.

Methodology
The rock specimens used in this research are sandstone outcrops from the Air Benakat formation which are exposed along the Lematang river in Lahat, South Sumatera (Figure 1).Prior to taking outcrop rock samples, a literature review of the geological map of Selamat Lahat, South Sumatra was carried out to ensure that the outcrop rock samples taken were sandstone from the Air Benakat formation (Figure 2).X-Ray Diffraction (XRD) measurements were carried out to observe the changes in rock samples mineral composition before and after being treated with water + CO2 immersion.Rock samples with dimensions of 0.5 -1 cm × 0.5 -1 cm × 0.3 -0.5 cm has been prepared as shown in Figure 3.Those samples would be applied different treatments as shown in Table 1.

A4
Soaked by water + CO2 (14 days) Flow-through experiment has been setup (Figure 4) to soak the rock sample with water + CO2.pH of water is measured over time (3 -7 -14 days).Before rock samples are soaked into flow-through chamber, the dry mass is measured.After several days, the certain rock sample is taken out and dry mass is measured again to observe weight loss of soaked rock samples.

Results and Discussion
XRD has been carried out on A1 rock sample as a baseline of mineral composition before soaked by CO2 + water.It showed that dolomite (CaMg(CO3)2) was the dominant mineral composition (79%) followed by quartz (16.9%) as shown on Figure 5. Figure 6 showed the pH of water + CO2 over time.It is observed that pH is ranging from 4.5 -4.9 (average: 4.75), proving that the reaction between CO2 and water has formed carbonic acid as shown in Eq. ( 1).Dry mass (before & after soaked with CO2 + water) has been measured (Figure 7 & Figure 8).Figure 9-Figure 10 showed visual observations of some mineral dissolution on A2 samples.It proved that some part of rock samples has been dissolved due to chemical reaction between dolomite and carbonic acid.Figure 11-Figure 13 showed the XRD results of A2, A3 and A4 samples (after soaked by CO2 + water for 3, 7 and 14 days).Table 2 showed the XRD results summary.On A2 & A3 samples, decreasing of quartz composition were observed (from 17% to ± 15%).On the other hand, A4 sample showed the decreasing of dolomite composition (from 79% to 75%).XRD results were not conclusive yet to determine which minerals were significantly affected by carbonic acid, since treated samples showed different mineral composition alteration trend.SEM-EDS and thin section were suggested to give better insight of carbonic acid effect on pore structure and mineral composition alteration.However, visual observations showed the indication of mineral dissolution (unfortunately, the rock flakes are not sufficient to be tested).It can be concluded that Air Benakat reservoir was vulnerable to sanding phenomena due to CO2 injection.

Conclusion
The investigation of sanding effect on the Air Benakat reservoir sample (dominated by dolomite and quartz) in South Sumatera due to CO2 injection has been conducted using flow-through experiment and observations.The changes in rock's pores due to the dissolution process were also clearly observed from dry mass measurements, visual observations and XRD analysis during the injection process of carbonic acid fluid into rock samples from time-to-time measurement.Those measurements concluded that Air Benakat reservoir sample was vulnerable to sanding phenomena due to CO2 injection.

Figure 4 .
Figure 4. Flow through experiment setup

Table 1 .
Rock samples treatments

Table 2 .
XRD results summary