Experimental study of the effect of thermal stresses on the failure of a carbonate rock, southern Iran



Productivity of an oil well is an important parameter in any oil field development. Low production rate of the wells, impose more production wells drilling which directly jeopardizes the economical feasibility of the project. Oil production rate in some of the wells is very low due to low permeability of the reservoir rock. Several practice like hydraulic fracturing and acid fracturing is very common in the oil industry. In these methods high pressure fluid (or acid) is injected into the reservoir and when the pressure exceeds the rock strength, it starts to fall apart. Thermal cracking is one of the methods used indirectly to increase rock mass permeability and consequently oil production. Thermal stress induced by injecting cold fluid into the reservoir is actually part of the stress needed to break down the reservoir rock during hydraulic fracturing process. If this exceeds the critical magnitude, the rock will fail leading to permeability and productivity enhancement. Effect of thermal stress in enhancement of injection well during water flooding process and wellbore stability has been investigated and reported in several papers, however thermal stress as a method is not addressed and reported elsewhere.
In this experimental study effect of temperature gradient and induced thermal stress was investigated on one of the Iranian carbonate oil reservoir. The field is a carbonate reservoir with low permeability less than few mili-darcy. Well production is very small as low as 300 stb/day in some wells which makes the field a good candidate of stimulation job. Several samples have been taken from the reservoir and prepared for the experiments. The carbonate rock of the reservoir is very heterogeneous and contains layers of clay. Initial study on thin section shows that heterogeneous texture of the reservoir rock has the potential of thermal stress concentration. Three categories of fractures were distinguished in the reservoir rock which could be considered as weak surface in the process. Numerical simulation performed with FLAC simulator to understand how heat propagates with time in the rock sample. An experimental set up was designed for cooling the rock samples. The rock sample heated up to reservoir temperature which is almost 90oC and contained with a glass cap on its top. Hot water was circulated through the glass cap to keep the sample in the reservoir temperature during the experiment. Then the sample was cooled from one side to study the effect of temperature on the rock sample. The samples were CT scanned along and perpendicular to its axis before and after the experiment and each section was carefully analyzed for possible fracture. Experimental results showed that thermal cracking is feasible by imposing 90 C degrees temperature variation. Experiments performed on four samples of which two showed induced tiny fractures. The mini-fractures were mostly in the middle part of the sample which is related to temperature profile and induced thermal stress front inside the sample. Our experiment shows that thermal stress effect could cause fracture initiation in the sample and may be used as a driving force of fracturing in conjunction with hydraulic fracturing process for well stimulation. The results also indicate that thermal stress is important and should be considered in process like wellbore stability.