Hydraulic fracturing, which is also known as fracking, involves the high-pressure injection of fluid into a well passing through a given subsurface in which oil, gas and petroleum reservoirs are located in order to create cracks in the rock formations through which fluids including the oil, natural gas and petroleum can flow more freely. In general, the target reservoirs are typically porous sandstones, limestones or dolomite rocks. However these reservoirs also include what is termed unconventional reservoirs that include shale rock or coal beds. Also included in the subsurface are clays and kerogen filled pores in unconventional reservoirs. The effectiveness of hydraulic fracturing in a given subsurface depends on factors including the mineralogy of that subsurface and the presence of free silicon.
Free silicon is sourced by the dissolution of planktonic organisms, from residual silicon from pore fluids or from silicon released during weathering. It is amorphous and not mineralized into silicon dioxide crystals. The presence of free silicon within siliciclastic and calcareous mudstones can create a rigid high modulus framework that has the potential to mitigate the adverse effects of high clay content with respect to drilling and completions and may ultimately lead to better hydraulic fracturing and improved production.
Current analytical techniques, however, do not fully quantify or correctly interpret free silicon phases within certain lithologies, because the silicon may not be a mineralized species. The free silicon can be either an amorphous silicon dioxide phase or silicon ions locked within interstitial crystal lattice spaces. These forms of silicon prove to be difficult to determine using classical techniques such as traditional X-ray diffraction (XRD) and X-ray fluorescence (XRF), which are based on the fact that the matter to analyze is in a crystalized form.
In XRD, the analytical methodology measures the bond dimensions of crystal lattices. An incoming monochromatic polarized X-ray beam irradiates a sample surface, interacts with atoms located in different crystal layers and reflects back, away from the surface. Traditional scanning of a sample through different angles provides an XRD trace containing peaks corresponding to different angles that relate to constructive interference conditions from certain mineral species and certain lattice dimension. It is therefore possible to interpret which mineral species are present within the sample and ultimately to determine a quantitative estimate of their relative mass percent abundance.
However, in the case of free silicon, if the silicon occurs as an amorphous, i.e., poorly crystalline, silicon dioxide species. The lack of a crystal structure makes it very difficult for XRD to detect the presence of this material or to determine the amount of this material in the sample. The same problem occurs if the free silicon exists as inclusions locked within interstitial crystal spaces, as this silicon will not register on the XRD trace as a unique peak. These limitations of XRD analysis can result in overestimated quantities of highly crystalline materials, such as quartz, due to the underestimation of poorly to non-crystalline species, such as amorphous silicon and some clays.
Unlike XRD, XRF is used for the determination of the elemental composition of a sample, is not sensitive to the location of the elements in the sample and has a relatively good detection limit. XRF irradiates samples with a monochromatic X-ray beam and measures the resultant X-ray fluorescence induced by the incident beam. The induced fluorescence from the sample material is generated by the interaction between the incident X-ray beam and the various outer shell electrons that triggers the transition of an electron from a higher energy shell to fill the free space left by the ejected electron from the outer shell. This transition of a high energy electron toward a lower energy shell is coupled to the emission of an X-ray photon whose energy corresponds to the difference of energy between the two shells. The energy of the photon is therefore specific and allows identifying the type of atom present in the sample by analyzing peaks that represent a given element and the amplitude of those peaks, which indicate the amount of that element.
However, the base output data from XRF is the elemental composition of the sample, not its mineralogy. Methods to obtain the mineralogy using mass balance equations generally work when the lithology is well known and when dealing with a fully mineralized sample. However the mass balance process does not allow for “left-over” elements, meaning that any excess silicon in the sample such as free silicon, which cannot be assigned to complex silicates, ends up being assigned to quartz. As a result, in samples containing silicon as free ions in interstitial crystal spaces, the resultant bulk mineralogy will generally overestimate quartz due to this limitation, and leave the free-silicon unresolved.