Summary In well stimulation operations, the ability to sustain long-term conductivity of hydraulic/acid fractures defines an efficient and effective hydrocarbon production operation. However, it is challenging to keep the fracture conductive in the soft and weak carbonate formations due to many challenges. For example, the plastic deformation of rocks causes proppant embedment or asperities failure, resulting in fracture conductivity reduction. Consolidating chemicals, particularly diammonium hydrogen phosphate (DAP), have shown to be effective in rock consolidation and could reduce the decline in fracture conductivity if applied to carbonate formations. The previous research tested DAP at ambient conditions, whereas this work involves studying the hardening properties of DAP at reservoir conditions. The solutions with two initial concentrations (1 and 0.8 M) were tested at 77°F (ambient), 122°F, and 176°F. Furthermore, a post-treatment analysis was conducted to compare the performance of the chemical under different conditions. The analysis included understanding the changes in carbonate rocks’ (limestone and chalk) hardness (impulse hammer test and indentation test), porosity (helium porosimeter), permeability (steady-state and unsteady state nitrogen injection), and mineralogy [X-ray diffraction (XRD) and scanning electron microscopy (SEM)]. Results demonstrated that both rock lithologies reacted efficiently with the DAP solution, presented in terms of the noticeable improvements in their hardness. The elevated temperatures positively affected rock hardness, leading to a more than 100% increase in hardness for most samples. After obtaining successful results from experiments at various temperatures, the pilot American Petroleum Institute (API) conductivity experiments were conducted, testing the conductivity sustenance through the rock hardening concept. Preliminary API conductivity experiments have demonstrated that treated rock samples with DAP provided higher conductivity values than the untreated samples at high stresses. The results shown in this study provide a good foundation for further studies on the implementation of DAP in actual acid/hydraulic fracturing field operations.

No abstract

Chemical consolidation of rocks is a common practice in the petroleum industry. Usually, this technique is applied to sustain production from unconsolidated or weak sandstone formations and to avoid collapse from aging wells. Recent research has shown the significance of rock hardness in sustaining long-term conductivity of hydraulic fractures in carbonate formations. There are some notable attempts to adapt the techniques used in the cultural heritage industry for strengthening carbonate formations. Moreover, a novel carbonate rock strengthening methodology was developed that involves the mineral transformation of calcite to harder minerals. This work describes the existing techniques for carbonate rock consolidation and provides extensive research on chemicals that are (or can be) used as carbonate rock strengthening agents. Furthermore, this review provides the reaction mechanisms associated with a range of proposed chemicals. Also, laboratory methods that can be used to assess post-treatment hardness and investigate the changes in rock mineralogy are summarized. Finally, the review discusses chemicals with the prospect of their application in petroleum field operations.

Hydraulic fracturing operations involve generating conductive pathways in low permeability formations to maximize hydrocarbons production. One of the main challenges is maintaining fracture conductivity under high closure stresses, especially in soft formations. However, long–term fracture conductivity can be sustained through fracture surface hardness improvement. This research targets the strengthening of carbonate rocks via the transformation of calcite into the harder hydroxyapatite mineral. In this study, limestone, chalk, and dolomite rock specimens were treated with 1M solution of diammonium phosphate (DAP) for 3 days at room temperature conditions. Rock samples’ hardness was measured by indentation (Brinell hardness) technique before/after the treatment to assess the strengthening effect of DAP. The changes in the mineralogy in treated samples were studied by SEM-EDS technique. The formation of phosphate minerals was achieved in treated samples, and they were clearly seen in the SEM images. The results have shown that both limestone and chalk samples reacted strongly with DAP solution, which was expressed in terms of rich abundance in newly formed minerals inside rock specimens. The reaction between dolomite and DAP solution was observed to be weak which resulted in generation of isolated phosphate minerals. The formed minerals were identified as hydroxyapatite (5 hardness in the Mohs scale) after comparing their morphology with other phosphate minerals reported in the literature. Treatment of the rocks by DAP solution resulted in improvement of their strength. The Brinell hardness of the chalk specimen was increased by 30% after the treatment, whereas in the case of the limestone sample, a 13% increment in hardness was achieved. The proposed carbonate rock strengthening technique can be applied in hydraulic fracturing It is intended to solve common soft formations problems (e.g., asperities failure, proppant embedment) causing acid/propped fractures’ conductivity reduction.

Hydraulic fracturing is applied in tight formations to create conductive paths within the reservoir. However, the conductivity of the created fractures declines with time due to the closure stresses. The decline is sharp in soft formations because of proppant embedment and fracture surface asperities failure. The improvement in fracture surface hardness can mitigate the abovementioned challenges and sustain the fracture conductivity. This research targeted enhancing carbonate rock's hardness by forming minerals harder than calcite. Carbonate rocks, namely dolomite, limestone, and chalk, were treated at ambient temperature conditions by immersion into the aqueous solutions of NaF and ZnSO4 with a concentration of 0.1M. During treatment, the solution was sampled to monitor the changes in ion concentration and estimate the reaction kinetics by ICP - OES and IC devices. The hardness of rock samples was measured by impulse hammering technique before and after the treatment. The changes in rock's mineralogy and elemental content were studied by XRD and SEM imaging. The permeability of rocks was estimated by the steady-state gas injection method. The formation of smithsonite (ZnCO3, Mohs scale hardness - 4.5) and fluorite (CaF2, Mohs scale hardness - 4) was achieved in the reaction of calcite (CaCO3, Mohs scale hardness – 3) with ZnSO4 and NaF, respectively. Chalk and limestone reacted efficiently with both solutions; however, the dolomite reaction with solutions was feeble. XRD detected the newly formed smithsonite minerals, and it was observed in SEM images that minerals formed an interconnected net in chalk and limestone specimens. In dolomite samples, the minerals formed isolated gatherings that were sparsely located on the grains. The treatments caused the improvement of the rock specimen's hardness. 0.1M solution of NaF was not effective in strengthening the rock samples (only chalk sample experienced 6.7% improvement in hardness) because of low concentration of the solutions used; however, treatment resulted in negligible changes in permeability of the samples. In contrast, Young's modulus of limestone and chalk treated by ZnSO4 increased by 17% and 21%. Permeability of rocks treated by ZnSO4 reduced drastically, most likely due to the formation of gypsum as a byproduct of the reaction. This research presents a method for carbonate rock hardening via the transformation of parent calcite into harder minerals. It explains its possible application in the petroleum industry to sustain the conductivity of propped/acid fractures. The proposed technique will help to mitigate fracture conductivity decline due to proppant embedment and asperities failure issues that are especially severe in soft formations.