Investigation of Mechanical Properties of Shale Rock in Qassim Region , Saudi Arabia

An investigation into the mechanical properties of shale rock from Qassim Province, Saudi Arabia is presented in this paper. Uniaxial compression test, Schmidt hammer test and porosity estimation were carried out. Regarding the compression test, it was found that the strength ranged from 1.98MPa to 8MPa and the strain ranged from 0.53% to 2.5%. Regarding the Schmidt Hammer test, it was found that the rebound values ranged from 22.4 to 25. The measurements of volumetric porosity indicated that the porosity in the shale rock ranged between 19.12% and 24.31%. All the values determined in this project match well with the published values of other studies about shale rock. Keywords-shale rock; mechanical properties; Qassim province; Saudi Arabia


INTRODUCTION
Shale oil and gas production from organic rich shale formations is a growing area of technical interest in oil and gas exploration.Long horizontal wells with hydraulic fracturing are required to bring economic production from shale gas reservoirs.Since crack propagation in hydraulic fracturing occurs under high strain rates, it is important to understand the fracture behavior of shale rock and its mechanical properties.The properties of shale rock that are needed in order to be able to design the hydraulic fracturing systems include: compressive strength, hardness, porosity, modulus of elasticity, Poisson's ratio, fracture toughness and permeability.Rocks in general can be classified as: igneous rocks, sedimentary rocks, and metamorphic rocks.Sedimentary rocks are made when products of weathering are subjected to transportation by water, winds or deposition and subsequently are compacted or consolidated.Some examples are sandstone, shale, conglomerate, breccias, limestone, and coal.Minerals forming sedimentary rocks are kaolinite, illite, smectite, hematite, rutile, corundum and so on.Shale is basically a sedimentary rock with fine grains, composed of mud which is a mixture of flakes of clay minerals and small particles of other minerals, mostly quartz and calcite.
Numerous experimental and theoretical investigations have demonstrated how mechanical properties in sedimentary rocks are affected by porosity [1], clay content [2], overburden stress and pore fluid [3].Further understanding needs to be developed on how these parameters control rock strength.The aim of this paper is to test the mechanical properties of Saudi shale rock from Qassim province, under static loading.Rock samples were taken from shale formations in Qassim region, Saudi Arabia.Uniaxial compression test, Schmidt hammer test and porosity estimation were carried out.

A. Uniaxial Compression Test of Shale Rock
Weak rocks are defined by their ultimate compressive strength (UCS) in the range of 0.25 to 25MPa [4].A good strength limit for weak rocks may be 20MPa because rocks weaker than this behave differently when sheared [5].Sedimentary rocks such as sandstone, siltstone, shale, claystone or mudstone, clay-shale fall under the classification of weak rocks [6,7].The prime factor affecting the strength of weak rocks is porosity.Commonly, high porosity gives low strength and vice versa.Table I summarizes some available strength data for sandstone and shale.It can be seen that higher porosity leads to weak compressive strength.

B. Schmidt Hammer Test
Schmidt Hammer test is commonly used for testing different rocks and has a strong correlation with UCS through numerous empirical equations.Schmidt hammer rebound (R) values were directly used in the analysis and were not converted to UCS, since there is no standard conversion designated for shale.Table II shows the range of R values for Sevier and Rome shale [9].

C. Porosity
Porosity is defined as the fraction of a rock occupied by pores.It is a static property which can be measured by various methods in the absence of fluid flow.To find effective porosity fluid flow is required to ascertain whether the pores are interconnected.Table III presents some data on average porosity.Nearly all the measurements were made at room temperature and pressure of 1 atmosphere [10].

A. Specimens Preparation
The cutting of samples was completed in the Civil Engineering lab at Qassim University, using its sample preparation machine.It took five days to complete prepare the specimens to finally obtain only 20 proper samples out of 40. Figure 1 shows the shale samples before they were cut into cubical shapes.The specimens were cut as 50mm×50mm×50mm cubes as per ASTM C 109/C 109M.There may be some errors in the dimensions due to the difficulty in precision cutting of soft rocks.Also, there is a maximum expected dimension error of ±2mm.Samples before cutting

A. Uniaxial Compression Test
The laboratory uniaxial compressive strength is the standard strength parameter of intact rock material.An MTS universal tensile testing machine used with capacity of 25kN was used.Table IV shows the results of the compression test.The average strength of the 4 samples used is 2.5MPa.Table IV also shows the strain values of the 4 samples.The maximum is 2.5% for sample 3 and the minimum is 0.91% for sample 1, while the average is 1.46%.It is important to compare the results with previous ones, to observe and discuss their differences.Table V represents the uniaxial compressive strength of shale rock and its associated porosity, from [5], in comparison with Qassim shale results investigated in this paper.The average porosity of Qassim shale was 22.54%.The average UCS of Qassim shale was 2.5MPa which is well within the range of internationally published values as shown in Table V.

B. Schmidt Hammer Test
Three samples were tested by Schmidt hammer.The dimensions of the samples were 50mm×50mm×50mm.An Ntype Schmidt hammer with an impact energy of 2.2N.m was used to measure rock hardness.Table VI shows pictures of the three samples before and after the test.It also shows the rebound and strength values obtained.The maximum rebound value was in sample 2, where sample 3 had the minimum value.Note that the empirical relation used to estimate the compressive strength in [11] could not be valid for the specimens used in this test.Tables VII-VIII show the results of this study in comparison with previously published results [9].

C. Determination of Porosity
Mathematically, porosity can be defined by ( 1): where ф: porosity (%), V b : bulk volume (cm 3 ), V p : pore volume (cm 3 ), and V m : matrix volume (grain volume) (cm 3 ).V b of the sample was measured by the volumetric displacement method.
V m was measured by crushing the sample to grain size and immersing it in a container filled with water.Substituting the results in (1) we have the results shown in Table IX:   Uniaxial compressive strength was tested for 4 different samples, ranged from 1.98MPa to 5MPa and with an average of 2.5MPa, while the strain ranged between 0.91% and 2.5% and had an average of 1.46%.The Schmidt hammer test results found that the rebound values ranged from 22.4 to 25. Displacement method was used to determine the Qassim shale rock porosity, the results were 19.12%, 24.15% and 24.31% and the average porosity was 22.53%.The samples for this study were taken from a cement quarry.In future, samples can be derived from the drilling core, because that would give more meaningful results for shale gas exploration.Another suggestion is to calculate shale rock's modulus of elasticity and Poisson's ratio.Schmidt hammer can be done with a lighter Ltype Schmidt hammer, which has impact energy 0.735N.m.In addition, impact tests can also be conducted.

TABLE I .
COMPRESSIVE STRENGTH AS RELATED TO POROSITY

TABLE VI .
SCHMIDT HAMMER TEST RESULTS

Table X
[10]s the porosity results comparison of shale rock in current study and in[10].It can be seen that porosity measured values lie within the range of other published values.