| Title | Comparative Review of Methylene Blue Analysis Procedures in Nag-67 Redrill, Tiwi Geothermal Field |
|---|---|
| Authors | I.G. Segura, L.D.A. Guidote, V.N.G. Ejera, C.M.M. Estrella, J.N.R. Crisostomo, and A.A. Calibugan |
| Year | 2017 |
| Conference | New Zealand Geothermal Workshop |
| Keywords | methylene blue test, smectite, clay, Tiwi |
| Abstract | Methylene blue (MeB) analyses of clay samples from selected intervals of Nag-67 original hole and redrill leg were conducted during the 2016 Nag-67 redrill workover. This was done to establish local alteration patterns in the outfield Naglagbong injection sector, which is critical in determining the setting depth of the 13-3/8” casing string to ensure complete isolation of the argillic zone and guarantee adequate reservoir pressure containment. MeB titration tests were conducted in two different sample preparations; one utilizing a heat-treated, acidified clay suspension and another using an EDTA-treated suspension. Results from the method utilizing H2SO4 suspensions were found to be more difficult to replicate and appear to be overestimating the smectite content. This is potentially due to several limitations in the experimental set-up, contamination of archived Nag-67OH samples and subtle differences in acid and heat treatment damaging parts of the clay structure. On the other hand, titrations of EDTA-treated suspensions provided clear and consistent endpoints. The latter method revealed two separate smectite peaks within the clay-rich zone identified through the microscopy of rock cuttings. The peaks are encountered at 2820 – 2920 ft-MD and at 3400 – 3500 ft-MD. The interval starting from the first peak until the second smectite peak can be considered to be part of the argillic zone. From the second smectite peak at 3400 – 3500 ft-MD, the apparent surface areas of succeeding samples drop and taper off with depth, marking the increasing prevalence of more thermally-stable, non-expanding clay minerals such as illite. This signals the entry to the transition zone, and is the preferred depth for setting the bottom of 13-3/8” string. |