The Uniqueness of Resistivity Logs: G.B. Asquith

Log analysis pre-1958 was done using only resistivity logs and is called the era of "OLD" E-Log Analysis. The log analyst was able to develop quantitative methods using only Rt, Rw, Rxo, and Rmf. What makes the resistivity logs unique: 1.) Productive Potential can be based on quantitative methods using only resistivity logs, and 2.) only the deep resistivity logs record measurements outside the invaded zone and to obtain Rt resistivity must be measured beyond the invaded zone. The analysis of a Permian 3RD Bone Spring Sandstone and Wolfcamp [WolfBone] well in southeast New Mexico is used to demonstrate the relevance today of the "OLD" log analysis methods.

First is the "estimation" of resistivity porosity using the Rxo Log PHIrxo = (Rmf/Rxo)^0.5. Unconventional reservoirs like 3rd Bone Spring Sandstone and Wolfcamp have dual pore systems consisting of small (5-750nm) oil-wet organic pores and much larger (30-2000nm) water-wet mineral pores (Loucks and others, 2010). PHIrxo is a measure only of the larger water-wet mineral pores and CBW. The accuracy of PHIrxo is affected by residual hydrocarbons and clay bound water.

Second is Bulk Volume Water [BVW]. Because there are no porosity logs a traditional Buckles Plot [PHI vs. Sw] cannot be used. A new BVW Method is required called DBVW (Henderson and Asquith, 2023). DBVW is the difference between BVW's at different depths with the difference then plotted versus depth. DBVW data of a reservoir at Swirr [Krw = 0.0] will be constant when plotted versus depth. A quantitative number for DBVW variation is expressed in the equation DBVW(var.) = S[(ABS(DBVW)]/N. The DBVW of the 3rd Bone Spring Sandstone has the least amount of vertical variation [DBVW(var.) = 0.0018] compared to Wolfcamp A [DBVW(var.) = 0.0037] and Wolfcamp B [DBVW(var.) = 0.0025] indicating that the 3rd Bone Spring Sandstone should have the lowest water-cut. The BVW of the Wolfcamp B is very high [BVW >0.05] compared to the 3rd Bone Spring Sandstone [BVW = 0.02] and Wolfcamp A [BVW = 0.03] indicating that the Wolfcamp B has a high clay content and is potentially a non-reservoir.

Third is a method called Moveable Hydrocarbon Index [MHI] which is the ratio of Sw to Sxo [Sw/Sxo = [(Rxo/Rt)/(Rmf/Rw)]^0.5]. Using a Sw/Sxo cutoff for conventional sandstone reservoirs of <0.7 on the WolfBone data resulted in the following: 3rd Bone Spring Sandstone [342.5ft. Sw/Sxo<0.7], Wolfcamp A [119.5ft. Sw/Sxo<0.7], and Wolfcamp B [20.5ft. Sw/Sxo<0.7]. The MHI data using a Sw/Sxo cutoff of <0.7 indicated that of the three WolfBone zones the 3rd Bone Spring Sandstone is by far the superior reservoir, and the Wolfcamp B is a non-reservoir.

Fourth is a method to determine Mobile OOIPstb/ac. using Rt, Rw, Rxo, Rmf and m = 2.0. The method is called the Y – Method (Doll and Martin, 1954) where Y represents the amount of oil per unit volume displaced by mud filtrate during invasion. The equations for the Y – Method and Mobile OOIP are Y = (Rmf/Rxo)^0.5 – (Rw/Rt)^0.5 and Mobile OOIPstb/640ac. = S[(7758*Y*0.5'*640)/1.4]. The Mobile OOIP for the three zones are 3rd Bone Spring Sandstone [21.4], Wolfcamp A [13.8] and Wolfcamp B [2.5]. For comparison the Mobile OOIP based on NMR BIN Porosity > 10ms for the three zones are 3rd Bone Spring Sandstone [18.0], Wolfcamp A [14.6] and Wolfcamp B [1.6]. Mobile OOIP [Y-Method] moveable oil only from the invaded larger water-wet mineral pores CBW is excluded. These larger water-wet mineral pores are important to oil production in unconventional reservoirs (Lewis and Rylander, 2019).

Using only Rt, Rw, Rxo, Rmf and m=n=2 the following was determined: 1) 3rd Bone Spring Sandstone lowest water-cut, greatest thickness of Sw/Sxo<0.7, and highest amount of Mobile OOIPstb/640ac., 2.) Wolfcamp A higher water-cut, less thickness of Sw/Sxo<0.7, and a lessor amount of Mobile OOIPstb/640ac., and 3.) Wolfcamp B high clay, very low both Sw/Sxo<0.7 and Mobile OOIPstb/640ac. potential non-reservoir. The well was a vertical completion [WolfBone] with an IP of 332bopd+177mcfgpd+447bwpd. Cum. Prod. [2012-2017] 95,533bo+57,428mcf+201.026bw.