The Synergy Between Borehole Imaging and Geosteering

The quest to gain more knowledge of the subsurface and to reduce uncertainty in the interpretation of subsurface data has been an age-long effort in the oil and gas industry. To achieve this, asset owners use tools with improved resolution, utilize different types of logging tools and integrate the interpretation from these logging tools. This paper will review some projects where data from borehole imaging tools were used to support geosteering decisions and to gain more knowledge of reservoir structure. Borehole images are logs based on the circumferential measurement of a petrophysical parameter along a borehole wall. Logging-while-drilling borehole images can be used for structural, sedimentological and petrophysical analysis. These near-wellbore analyses contribute greatly to the success of most geosteering jobs. Geosteering is a process used in placing high-angled and horizontal wells in subsurface intervals of interest. It involves the use and integration of data from varied sources. This paper will show different scenarios, in different depositional environments, where borehole imaging supported the geosteering process and how geological interpretations from geosteering brought more clarity to borehole imaging analysis. Examples of these will highlight the stratigraphic relationship between geological structures and wellbore trajectory, detection of subsurface structural discontinuities, primary sedimentary structures, and the interpretation of complex geological structures. This paper will broaden our understanding of the applications of borehole imaging and how it integrates with geosteering in achieving oil and gas well objectives.

Spatially resolved wettability measurements using nmr wettability index

Wettability is a crucial petrophysical parameter for determining accurate production rates in oil and gas reservoirs. However, industry standard wettability measurements (Amott Test and USBM) are expensive and time consuming. It is known that NMR response varies as a function of wettability change in rock core plug samples. This information was used to develop an NMR wettability index (NWI) based on T2 distributions. This NWI is capable of measuring changes in wettability as a function of oil/water saturations unlike traditional methods which are based on measurements at Swi and Sor only. In addition, these oil/water saturations are determined without the aid of any special oil or brine, such as D2O. This allows the NMR method to nondestructively monitor changes in wettability in real time (i.e. during a flooding experiment or an aging procedure). In this work, we have coupled this T2-based NWI to spatially resolved T2 NMR measurements to monitor changes in wettability and saturation along rock core plugs. In order to derive an NMR wettability index, NMR T2 spectra of 100% brine saturated, 100% oil saturated, bulk oil and bulk brine are needed. These spectra are then mixed to give a predicted T2 spectrum which is compared (via a least squares fit) to a T2 spectrum recorded from a sample partially saturated with both water and oil and whose wettability is to be determined. For initial testing, three sandstone samples were employed along with 2% KCl brine and dodecane. To achieve sample states of mixed wettability, 100% brine saturated samples had dodecane pushed into them via centrifugation. Centrifugation at different speeds resulted in samples of varying bulk and spatial wettabilities from which NWI parameters and oil/water saturations were determined. The bulk wettabilities were compared to measurements done using the standard Amott test and oil/water saturations were confirmed by repeating experiments using NMR invisible D2O.

New Relationship Between Resistivity Index and Relative Permeability

Relative permeability as an important petrophysical parameter is often measured directly in the laboratory or obtained indirectly from the capillary pressure data. However, the literature on relationship between relative permeability and resistivity is lacking. To this end, a new model of inferring two-phase relative permeability from resistivity index data was derived on the basis of Poiseuille's law and Darcy's law. The wetting phase tortuosity ratio was included in the proposed model. The relative permeabilities computed from the capillary pressure data, as well as the experimental data measured in gas–water and oil–water flow condition, were compared with the proposed model. Both results demonstrated that the two-phase permeability obtained by proposed model were generally in good agreement with the data computed from capillary pressure and measured in the laboratory. The comparison also showed that our model was much better than Li model at matching the relative permeability data.