NMR Wettability Index Measurements on Unconventional Samples

Dick, Michael (Green Imaging Technology) | Veselinovic, Dragan (Green Imaging Technology) | Green, Derrick (Green Imaging Technology) | Scheffer-Villarreal, Aimee (ConocoPhillips) | Bonnie, Ronald (ConocoPhillips) | Kelly, Shaina (ConocoPhillips) | Bower, Kathleen (ConocoPhillips)



Wettability is a crucial petrophysical parameter for determining accurate production rates in oil and gas reservoirs and may be especially impactful in predicting the extent of injected fluid imbibition and resultant drainage in the vicinity of hydraulic fractures within unconventional reservoirs. However, traditional industry standard wettability measurements (Amott test and USBM) often fall short when performed on unconventional samples. In this work, we adapt the existing T2-based NMR wettability index (NWI) measurement to unconventional samples in order to provide robust wettability measurements for tight rocks.


Wettability describes the affinity of a fluid to a solid surface and is dependent on rock properties such as mineralogy, aging, and brine and hydrocarbon composition. As a system always seeks to minimize surface energy toward equilibrium, whether a surface is hydrophobic (prefers to contact non-aqueous fluid molecules, usually of lesser polarity than water) or hydrophilic (prefers water) will determine the native state distribution of brine and hydrocarbon as well as the dynamic behavior of these saturations. It is well known in conventional reservoirs that wettability can greatly influence the character of relative permeability curves and production. Conventionally, water wet is the preferred state for petroleum exploration, as water will reside in the smallest pores and hydrocarbons in the larger pores and apertures, but many successful reservoirs have mixed (or intermediate) wettability. The tight pore structures of unconventional reservoirs are also sensitive to wettability controls, if not governed by them due to strong capillarity; however, the influence of wettability on matrix and matrix-fracture transport during and after hydraulic fracturing is not as well understood as in (or for?) conventional reservoirs. Learnings on the role of wettability in unconventional rocks may render useful information for the design of well completion and enhanced oil recovery strategies.

A wettability assessment such as NWI may assist with testing wettability states and controls in tight rocks in a quantitative matter. Recall that a wettability index of 1 is very water wet, −1 is very oil wet, 0 is neutral/mixed wet, and values close to 0 are weakly oil or water wet. This research demonstrates the utilization of the NWI technique on two twin sets of South Texas unconventional core plugs expected to have differing wettability due to significantly higher organic matter content in one of the sample sets. The samples, generally labeled sample 2-PX and 7-PX, are from the same well in producing acreage, but different lithological units. Sample 2-PX is a chalk and sample 7-PX is a marl; the latter has significant organic matter and clay content. Some basic petrophysical properties of these samples are listed in Table 1.