Acid Fluid Induced Water-rock Interaction During Diagenesis And Its Effect On The Sandstone Reservoir Porosity

An accurate prediction of the petrophysical properties in sandstone reservoirs is of great importance in petroleum exploration, development and production. Since diagenesis is one of the most direct factors affecting the reservoir quality, it is necessary to understand the effect of diagenesis on the evolution of reservoir petrophysical properties. Formation water in petroleum reservoirs is a complex solution, containing a variety of dissolved constituents. A series of water-rock interactions could inevitably occur because of a long-term contact of the formation water with the surrounding rocks during diagenesis. The interactions lead to not only the redistribution of chemical species in rocks and water, but also the dissolution and precipitation of minerals, resulting in changes in reservoir porosity and permeability. Investigation of the influences of water-rock interactions on the reservoir quality is of great importance for effective oil-gas exploration and development.There are numbers of preliminary studies on diagenesis using various methods, such as petrography, geochemical calculation, isotope analysis, numerical simulation, and so on. They can be generally classified in two categories. One is about diagenetic sequence analysis by field investigation and laboratory analysis work, whereas the other focuses on typical diagenetic evolution, such as carbonate and silicate rock, using computation and simulation methods. So far, there is rare research work focusing on water-rock interactions during diagenesis and their effects on reservoir porosity. With the rapid development of computer and digitization technology, simulation methods are increasingly improved. Numerical simulation of diagenesis can effectively simplify the heterogeneity of geological bodies and the complexity of the comprehensive process, so as to provide a quantitative evaluation of reservoir quality for oil and gas exploration. Existing basin models mostly focus on the geometric shape change or compaction and neglect water-rock interactions, which limit the exploration accuracy. Therefore, reactive transport simulation should be introduced to the diagenesis study. It would be helpful to investigare elements migration, evolution and redistribution during diagenesis.This study aims to investigate water-rock interactions under different burial conditions during continuous diagenesis and predict porosity evolution quantitatively by combining petrographic observations, laboratory experiments and numerical simulation methods. It is an excellent approach to ascertain the main factor of diagenesis, replicate the diagenetic process quantitatively, and evaluate spatialtemporal distribution of the reservoir porosity.Numerical simulation method is employed to investigate the influence of primary mineral heterogeneity on the diagenesis. Based on measured data from the Upper Cretaceous reservoir, a total of twenty-six 2-D models with various mineral compositions are constructed. Monte Carlo procedure is employed to mimic the heterogeneous mineral distribution regionally and six heterogeneous models at different scales are simulated. Results show that geochemical reactions and porosity reduction caused by different detrital mineral vary widely, especially for chlorite, anorthite and feldspar. The chlorite dissolution could promote ankerite precipitation. With the increase of anorthite content, there is less chlorite dissolution providing Fe2+ and Mg2+ for ankerite precipitation, which makes kaolinite main secondary minerals. The mineral heterogeneity causes similar distributions of the geochemical reactions and reservoir quality evolution. Therefore, the type, content and heterogeneity of detrital mineral influence water-rock reaction directly, and then lead to the heterogeneous distribution of reservoir porosity.To investigate effects of acetic acid on arkose dissolution under different burial diagenetic conditions, seven batch experiments are conducted over a wide range of reaction conditions. Medium-grained lithic arkose from the Yingcheng Formation in Songliao Basin is selected. Four factors are considered, which are temperature, p H, initial water/rock mass ratio and grain size. Results show that these four factors have significant but different effects on arkose dissolution. In a certain range, feldspar dissolution enhances with the increase of temperature, but declines when it exceeds a certain value. Different minerals/elements are affected by temperature variously. Silicon is the element most sensitive to temperature. Under acidic conditions, dissolution rates of most minerals increase as the p H decreases. Grain size and water/rock mass ratio mainly affect the reaction rate rather than mineral transformation. Feldspar minerals dissolve in different extent under various conditions, which causes the special development zone of secondary pores.The Cretaceous sandstone reservoir in Tarim Basin is investigated with reference to its reservoir property evolution during diagenesis. Six general diagenetic stages are recognised through petrographic, mineralogical and geochemical analyses. Water-rock interaction experiments are conducted under these six successive diagenetic stages, that is, compaction ? early diagenesis ? organic acid intrusion I ? high temperature/pressure ? organic acid intrusion II ? late diagenesis. A total of six models corresponding to the experiment are simulated. Finally, an extended model at geological time scale(from 30 Ma to present) is constructed after the parameters being validated. Results show that water-rock interactions vary greatly with the increase of temperature/pressure. Main diagenetic minerals are quartz overgrowth, calcite cement, ankerite cement and late kaolinite cement in sequence. The organic acid intrusion increases the reservoir porosity from 20.0% to 21.7%. In late diagenesis, the fluid is diluted and most minerals precipitate under alkalic environment, which decreases the reservoir porosity to around 11.4%. The combination of simulation and experiment could reappear diagenetic evolution process quantificationally.The Shihezi-8 Formation is a tight sandstone reservoir containing large number of tight gas. Eight 2-D models including three factors(i.e. mineral, solution, and CO2 intrusion) are constructed to analyze CO2-water-rock interactions and find out the formation condition of tight reservoir. A 3-D model with porosity, permeability and mineral heterogenity is simulated to investigate the heterogeneous distribution of porosity. Results show that the reservoir porosity is influenced by three factors simultaneously. With the increase of CO2 intrusion, porosity decreases more drafically. The calcium chloride type of solution is more vulnerable to densification because it is rich in Ca2+, which is a major carbon sequestration ion. Reservoirs containing Ca-rich minerals are more inclined to densify because their dissolution could release Ca2+. CO2 migrates along the high permeability channel in heterogeneous model, which leads to mineral dissolution and increases the porosity. Therefore, water-rock interaction aggravates the heterogeneity. Numerical simulation could show the heterogeneous distribution of porosity in space, which is beneficial to the evaluation and prediction of favorable reservoir.