Improved Wellbore Strengthening
Drilling operations of both conventional and unconventional oil and gas accumulations are becoming more challenging especially in deep-water operations. Due to the narrow mud weight window in offshore wells, a proper wellbore stability analysis is required for a cost-effective execution. Wellbore strengthening is an approach used to increase the fracture pressure of the rock, widen the mud window and consequently enhance well integrity and mitigate mud losses. nFluids has demonstrated the feasibility of wellbore strengthening in permeable formations using oil-based mud (OBM) containing specially prepared nanoparticles (NPs) combined with graphite. A significant increase in the fracture pressure was achieved and the predominant wellbore strengthening mechanism was identified.
Fracture pressure increase was quantified by carrying out hydraulic fracturing tests on 5 3/4” x 9” Roubidoux sandstone cores. A 9/16” wellbore was drilled, cased and cemented to simulate well conditions. As part of the test, overburden and confining pressure was applied on the cores to simulate a normal faulting regime. Two injection cycles were applied allowing 10 minutes of fracture healing between the cycles. Optimum NPs concentrations were established after a comprehensive experimental screening and a strong relationship between wellbore strengthening and mud filtration at high-pressure high-temperature (HPHT) using a filter press on ceramic discs was found. The sandstone samples were characterized by porosity, permeability, and tensile and all the samples showed very similar properties: porosity ~14.5%; permeability ~12.0 md; tensile strength ~2.3 MPa.
Hydraulic Fracturing Tests
The cores were tested in a hydraulic fracturing apparatus that applies overburden and confining pressure, and allows injection of fluid into the wellbore using syringe pumps. More than one injection cycle was carried out and all the injection parameters vs. time were tracked using a software. Wellbore strengthening was obtained using NPs in OBM and trends as a function of NPs and graphite concentrations were identified. The hydraulic fracturing tests involved eight blends containing calcium-based NPs, iron-based NPs, and graphite. Overburden pressure of 400 psi and confining pressure of 100 psi were applied to the cores. A fluid injection rate of 5 mL/min was used. The control samples (CS) with no presence of NPs and graphite was tested initially as a reference point. Two additional tests using graphite at 0.5 wt% (without NPs) and graphite at 2.0 wt% (without NPs) were conducted to serve as reference points for identification the real effect of NPs addition on blends.
Wellbore strengthening was successfully achieved by adding nanoparticles into drilling fluid and testing on the sandstone cores. As shown in Fig. 1, the fracture breakdown pressure was increased by up to 65%. An initial leak-off-pressure increase was believed to occur due to an effective filter cake formed around the wellbore preventing pore pressure increase. A strong match between wellbore strengthening and filtration at HPHT was found and this could lead to inferences about the physical phenomenon of the strengthening process. Overall, for a particular NP type, higher filtration was proportionally associated with a less significant wellbore strengthening. The hypothesis behind this relies on the fact that while it is true that some filtration is required for the carrier fluid to dehydrate and form an immobile mass into the formation, excessive mud filtration produces NPs agglomeration and rapid dehydration of the blend while traveling along the fracture. If this occurs, the particles will not have an effective transport medium to travel and deposit at the fracture tip. Optical microscopy, scanning electron microscope (SEM) and energy dispersive X-ray spectroscopy (EDX) analyses were conducted on the hydraulically fractured cores – the fractures were seen to be completely sealed from tip to wellbore. A 40 micron-seal containing homogeneously dispersed NPs and graphite was observed. In addition, a 300 micron-filter cake was developed around the wellbore also containing homogeneously dispersed NPs.
Figure 1 An impact of the 1st generation nForcer™ nanoparticle additive on the fracture breakdown pressure of Roubidoux sandstone cores.
Results demonstrated that wellbore strengthening is possible in sandstone cores using the nFluids’ NPs at low concentration in OBM. The predominant wellbore strengthening mechanism was identified through optical microscopy, SEM, and EDX analyses of cores post-testing. A strong match between wellbore strengthening and mud filtration at HPHT in permeable media was found and this allowed a better understanding of the phenomena behind wellbore strengthening. Tip resistance by the development of an immobile mass was identified as the predominant wellbore strengthening mechanism by ruling out the occurrence of other possible mechanisms and post-testing analysis. Some filtration was observed to be beneficial but high filtration impairs the wellbore strengthening mechanism. This was explained based on the hypothesis that the high-filtration blends that traveled along the fracture became quickly dehydrated and particles did not completely reach the fracture tip.
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