Systematic Evaluation of Rock Mechanical Behavior of Chalk Reservoirs in the Presence of a Variety of Water Compositions

Principally, the interaction between chalk surface and saline water is vital for reported subsidence, formation compaction (the decline in pore fluid pressure within a solid structure and increase in stress on formation rock), and enhanced oil recovery (EOR) in chalk reservoirs. To understand the mechanisms of rock-fluid interaction and the role of specific ions in seawater, rock mechanical tests combined with chemical analysis of effluents were performed on chalk outcrops using synthetic brine solutions. All the chalk cores were treated with strictly aqueous solution, which means no hydrocarbon involved in any stage of experiments from preparation to postprocessing (chemical analysis). The two objectives in the present study are the examination of diffusion and transport-controlled phenomena in the presence of different aqueous chemistry and the proposal of possible processes/explanations based on distinct experimental scenarios. The experiments supply information on chemical mechanisms in chalk water weakening and the effect of saturation and flooding with the synthetic brine solutions, NaCl, Na₂SO₄, MgCl₂, CaCl₂, and synthetic seawater without magnesium (SSW-1[Mg²⁺]). Two different thermodynamic conditions (flooding state) were applied to the compacted cores within the creep phase: flooding steadily through the core and bypassing the core, both at high temperature and high pressure. The method provided measurements to investigate the effect of flow rate on the chemical reactions between the rock and fluid. Yield stress level, bulk modulus value, and creep strain rate are three mechanical properties of samples that were measured and analyzed together with chemical analysis of effluents taken periodically during the creep phase of each experiment. The results subsequently provided information about weakening/strengthening behavior within a time-dependent period. Switching of flooding fluid from distilled water (DW) to the NaCl solution tripled the creep strain rate and produced calcium content equal to seawater concentration. Performing a similar action on a DW to Na₂SO₄ solution generated enhanced creep of close to a factor of three. In addition, adsorption of sulfate to the chalk surface was identified. Flooding solely with NaCl solution or Na₂SO₄ solution caused a 25 and 50% decrease in chalk mechanical strength compared with DW flooded cores, respectively. MgCl₂ solution flooding experiments generated cores with 10% higher mechanical strength compared with cores treated with Na₂SO₄ solution. The observation was confirmed by testing cores with SSW-1[Mg²⁺] in which relatively low hydrostatic yield and enhanced creep strain were observed compared with Na₂SO₄ solution testing. Chalk cores flooded with MgCl₂ solution showed 25 and 50% lower mechanical strength values compared with cores tested with NaCl and DW. Collected samples from MgCl₂ and SSW-1[Mg²⁺] flooding showed simultaneous processes of magnesium retention inside chalk and production of calcium. Changing the flooding state to bypass with Na₂SO₄ solution did not generate any difference in the creep strain rate, whereas a change of flooding state from bypass to flooding triggered production of calcium equal to one-third of the calcium content in seawater. However, an opposite observation was recorded when the flooding state changed when using MgCl₂ as a flooding fluid. Change of the flooding state from flooding to bypass created enhanced compaction in the case of MgCl₂. The observed trend was similar when changing from bypass to flooding in the case of testing with Na₂SO₄ and SSW-1[Mg²⁺]. The extensive experiments provided a foundation for analyzing the behavior of chalk brine in the presence of oil and building verified models to simulate the effect of flooding complex brine and seawater on the mechanical characteristics of chalk and predicting the chemomechanical behavior of chalk.