Brazilian Indirect Tensile Strength Research Articles

Splitting tensile strength of shale cores: intact versus fractured and sealed with ureolysis-induced calcium carbonate precipitation (UICP)

Ureolysis-induced calcium carbonate precipitation (UICP) is a biomineral solution where the urease enzyme converts urea and calcium into calcium carbonate. The resulting biomineral can bridge gaps in fractured shale, reduce undesired fluid flow, limit fracture propagation, better store carbon dioxide, and potentially enhance well efficiency. The mechanical properties of shale cores were investigated using a modified Brazilian indirect tensile strength test. An investigation of intact shale using Eagle Ford and Wolfcamp cores was conducted at varying temperatures. Results show no significant difference between shale types (average tensile strength = 6.19 MPa). Eagle Ford displayed higher strength at elevated temperature, but temperature did not influence Wolfcamp. Comparatively, cores with a single, lengthwise heterogeneous fracture were sealed with UICP and further tested for tensile strength. UICP was delivered via a flow-through method which injected 20–30 sequential patterns of ureolytic microorganisms and UICP-promoting fluids into the fracture until permeability reduced by three orders of magnitude or with an immersion method which placed cores treated with guar gum and UICP-promoting fluids into a batch reactor, demonstrating that guar gum is a suitable inclusion and may reduce the number of flow-through injections required. Tensile results for both delivery methods were variable (0.15–8 MPa), and in some cores the biomineralized fracture split apart, possibly due to insufficient sealing and/or heterogeneity in the composite UICP-shale cores. Notably in other cores the biomineralized fracture remained intact, demonstrating more cohesion than the surrounding shale, indicating that UICP may produce a strong seal for subsurface application.

Experimental Investigation of Shale Tensile Failure under Thermally Conditioned Linear Fracturing Fluid (LFF) System and Reservoir Temperature Controlled Conditions.

Linear fracturing fluid (LFF) provides viscosity driven benefits of proppant suspensibility and fluid loss control, and with the use of a breaker agent, flowback recovery can be greatly enhanced. Shale tensile strength is critical in the prediction of fracture initiation and propagation, but its behavior under the interaction with LFF at reservoir temperature conditions remains poorly understood. This necessitated an in-depth investigation into the tensile strengths of Eagle Ford and Wolfcamp shales under thermally conditioned LFF and reservoir temperature controlled conditions. Brazilian Indirect Tensile Strength (BITS) testing was carried out for the quantitative evaluation of shale tensile strength, followed by extensive failure pattern classifications and surface crack length analysis. The thermally conditioned LFF saturation of shale samples led to average tensile strength (ATS) increases ranging from 26.33–51.33% for Wolfcamp. Then, for the Eagle Ford samples, ATS increases of 3.94 and 6.79% and decreases of 3.13 and 15.35% were recorded. The exposure of the samples to the temperature condition of 90 °C resulted in ATS increases of 24.46 and 33.78% for Eagle Ford and Wolfcamp shales, respectively. Then, for samples exposed to 220 °C, ATS decreases of 6.11 and 5.32% were respectively recorded for Eagle Ford and Wolfcamp shales. The experimental results of this research will facilitate models’ development towards tensile strength predictions and failure pattern analysis and quantifications in the LFF driven hydraulic fracturing of shale gas reservoirs.

Effect of Rice Straw on Tensile Properties of Tailings Cemented Paste Backfill

It is important and difficult to improve the tensile strength of backfill material to ensure the stability of goafs. In this study, rice straw (RS) in fiber form is used to improve the tensile properties of cemented paste backfill (CPB). An orthogonal experiment was designed, Brazilian indirect tensile strength tests were conducted to test the tensile performance of RS fiber-reinforced cemented paste backfill (RSCPB) under different fiber content (1, 2, 3 kg/m3) and fiber length (0.8~1, 1~3, 3~5 cm), and the microstructure of RSCPB was analyzed with scanning electron microscopy (SEM). The results showed that, compared with the conventional cemented paste backfill (CCPB), the increase in tensile strength of RSCPB ranged from 115.38% to 300.00% at 3 days curing age, 40.91% to 346.15% at 7 days, and −38.10% to 28.00% at 28 days, and the strain was slightly reduced during the curing period. The tensile strength, strain, and percentage increase of the RSCPB compared to the CCBP did not show a monotonic pattern of variation with the RS fiber content and length during the curing period. The RSCPB samples fractured under peak stress, showing obvious brittle failure. In addition, sulfate generated from S2− in the tailings inhibits the hydration reaction, and generates swelling products that form weak structural surfaces, which, in turn, lead to a 28-day tensile strength and strain of RSCPB lower than those at 7 days.

Modification of the Brazilian indirect tensile strength formula for better estimation of the tensile strength of rocks and rock-like geomaterials

Rock mechanical properties are essential input in many design applications in petroleum, mining, environmental and civil engineering. The most reliable source for these properties is the laboratory tests. Among these important rock mechanical properties are the tensile strength. Tensile strength can be measured directly and indirectly. For rocks, the indirect tensile test is the most convenient in terms of samples preparation, testing requirements, and availability of appropriate standards. The most common experimental method used to indirectly estimate tensile strength of rocks is called Brazilian indirect tensile test. It is well documented in the literature that the Brazilian indirect tensile test provides over estimated tensile strength values compared to the direct tensile test. The objective of this work is the investigation of potential modification of the Brazilian indirect tensile strength formula by the analysis of the relevant published laboratory data.Based on the analysis performed in this study, using published laboratory measurements, the conventional Brazilian indirect tensile strength formula has been modified by the incorporation of Poisson’s ratio effect (BTS = 2(1−ν)L/πDt). The modified Brazilian indirect tensile strength formula was validated using another set of published laboratory data for various rocks and rock-like geomaterials other than that used initially in the modification process. Direct tensile strength values have been estimated; for various types or rocks; using the modified Brazilian indirect tensile strength formula with an average error of estimate as low as 8% compared to 32.6% before correction. Therefore, the modified Brazilian indirect tensile strength formula (BTS = 2(1−ν)L/πDt) is very promising and suitable for most rocks and rock-like geomaterials having Poisson’s ratio value lies between 0.10 and 0.45.

A new approach for understanding the mechanism of wellbore strengthening theory

To achieve fast, safe and economical drilling operations through formations containing natural fractures or vugs, all fractures and vugs must be perfectly sealed using proper fractures seal materials in a process called wellbore strengthening. Wellbore strengthening may elevate the overall tensile strength of the treated formation to a value approaches the intrinsic tensile strength of unfractured rock.Numerous studies have been conducted for testing fractures seal materials suitability for wellbore strengthening application in various formations. However, there is no solid and simple theoretical explanation on how wellbore strengthening process can recover the tensile strength and hence the fracturing pressure of the treated naturally fractured formation.In this study, test specimens cored from an artificial (building) sandstone bricks were used to measure the unconfined compressive strength (UCS), Brazilian indirect tensile strength (BTS), and establishing the multi-stage triaxial compression (MS-TCS) test. Core samples from the same sandstone were previously used for wellbore strengthening studies using crushed dates palm seeds. Crushed dates palm seeds succeeded to completely stop mud losses at a pressure equal to 6.9 MPa.Data obtained from unconfined compression, multi-stage triaxial compression, Brazilian indirect tensile, and wellbore strengthening tests were collectively used to develop a clear theoretical understanding of the mechanism of tensile strength recovery caused by wellbore strengthening effect.It was found that wellbore strengthening effect in sealing natural fractures and vugs is similar to the confining pressure effect in closing the induced shear fractures in the test specimen during a multi-stage triaxial compression test.For the tested sandstone, using crushed dates palm seeds as a fracture seal material has improved wellbore strengthening from zero tensile strength (complete loss of mud circulation) to 2.56 MPa tensile strength at which there was a complete fracture seal (zero mud loss). The recovered tensile strength (2.56 MPa) is approaching the intrinsic tensile strength (BTS = 2.7 MPa) of the same sandstone in its initial unfractured state.

Models to estimate the elastic modulus of weak rocks based on least square support vector machine

In many parts of the world, dams, hydroelectric power plants, tunnels, and highways have to be built in the regions where the weak rocks are dominant. In the solution of the above-mentioned geotechnical design problems, the elasticity modulus is an important parameter to be found. However, it is quite difficult to find the elastic modulus (E) values of highly porous low-strength rocks with standard laboratory tests. Therefore, researchers are trying to predict the elastic modulus of the rocks with new soft computing methods that use some simple inputs. In this study, the elastic modulus of the low-strength rocks was estimated by the least square support vector machine (LS-SVM) method. The properties of weak volcanic rock samples in different strength, welding degree, and geologies were analyzed. In all the prediction models, the tangent modulus of elasticity is used as a dependent variable, while the seismic P wave velocity (Vt), dry unit weight (ad), axial point load strength (Is50a), diametral point load strength (Is50d), and Brazilian indirect tensile strength (σt) parameters were used as independent variables. It was found that the LS-SVM method is a more effective tool compared with the multiple regression model in terms of prediction performance. Thus, the results show that if the elastic modulus of the rocks cannot be found directly by the experiment or test findings are suspected, the LS-SVM method may be a good alternative way to estimate the elastic modulus of the low-strength rocks.

Influence of Polypropylene Fiber Reinforcement on Tensile Behavior and Failure Mode of Tailings Cemented Paste Backfill

Tailings were used to prepare cemented paste backfill (CPB) reinforced with polypropylene (PP) fiber, and fiber content and fiber length were 0-20 % and 3-12 mm, respectively. The Brazilian indirect tensile strength tests, macrostructural and microstructural (SEM) failure mode analyses and nuclear magnetic resonance (NMR) tests were performed. The results demonstrate that PP fiber with high tensile strength can substantially enhance the tensile strength and ductility of CPB specimens. The fiber-reinforced CPB exhibits superior performance during the pre-peak and post-peak stages and can retain its residual strength after being broken. The fiber-reinforced CPB provided a higher secant Young's modulus and strain at peak stress than that of the unreinforced CPB. From the failure mode analysis, fiber was used to bridge the CPB microelements and combine these effectively with calcium silicate hydrate gelling. The governing failure mode of fiber is pulled out or pull off, through which fiber absorbs the tensile stress and energy. However, fiber is also believed to have confusion distribution in CPB because fiber cannot reach their full strain capacity, especially at the later curing stage. And the weak structural layers in the CPB can also be formed, and the higher tensile strength is not achieved with the additional fiber content or longer fiber length. The optimal fiber content in this study is recommended to be 0.15 %, and it's better to choose 6 or 9 mm fiber. Moreover, fiber can reduce the original porosity by 10.72 %, but only has a slight influence on the pore size. The decrease in porosity helps increase the tensile strength of the fiber-reinforced CPB.

An ANN Approach for the Prediction of Uniaxial Compressive Strength, of Some Sedimentary and Igneous Rocks in Eastern KwaZulu-Natal

The Uniaxial Compressive Strength (UCS) of intact rocks is an essential index of strength in rock engineering. Laboratory based direct compressive strength estimation may be problematic, as obtaining fresh samples is not always feasible. Thus, the aim of indirect methods index test such as point load index test, and empirical correlations with UCS of indexes like the Brazilian indirect tensile strength test, serve as an alternative for many geotechnical engineering projects. The aim of this paper is to propose a relationship between UCS and indirect tests or indexes for some sedimentary and igneous rocks in KwaZulu-Natal using the technology of artificial intelligence. These tests include the point load index (Is (50)) test and Brazilian Tensile Strength (σt), test. Block samples were collected in KwaZulu Natal, among these include sedimentary rocks (sandstones, siltstone, tillite) and igneous rocks (granitoids and dolerite). A back propagation artificial neural network was developed and trained in order to predict UCS. The input parameters were unit weight γ, (Is (50)), (σt), and lithology. The lithology was introduced in the neural network as a qualitative input parameter, in order to indirectly incorporate in the model the mineralogical content. Training results returned, R value of 0.99% for the training set, and R = 0.92% for the test set, which is conveying to the conclusion that the approach is valid and could be used, as an alternative indirect approach to UCS estimation.

Models to estimate Brazilian indirect tensile strength of limestone in saturated state

There are a number of methods for estimating physical and mechanical characteristics. Principally, the most widely used method is regression, but recently, more sophisticated methods such as neural networks have frequently been applied as well. This paper presents the models of a simple and a multiple regression and neural networks –types Radial Basis Function and Multiple Layer Perceptron, which can be used for the estimate of the Brazilian indirect tensile strength in saturated conditions. The paper includes the issues of collecting data for the analysis and modelling and an overview of the performed analysis with an efficacy assessment of the estimate for each model. After the assessment, the model which provided the best estimate was selected, including the model which could have the most wide-spread application in the engineering practice.

Effects of Testing Conditions on Intact Rock Strength and Variability

The strength of intact rock is a fundamental and integral part of many aspects of engineering and geologic practice. It is necessary to understand how testing conditions influence this strength and its variability, especially for use in reliability-based design. In this study, the effect of sample diameter and moisture content on the strength and its variability was examined in detail. Correlations are given for assessing the effect of sample diameter on the laboratory uniaxial compressive strength (qu), Brazilian indirect tensile strength (qt-Brazilian), and point load strength (Is), and the associated direct correlations among these tests. Correlations also are given for assessing the effect of sample moisture content on qu, qt-Brazilian, and Is. The statistics of data scatter suggest these correlations can introduce large uncertainties. The effect of sample diameter and moisture content on the coefficients of variation was examined subsequently, and it was found to be relatively minor.