Integration of Neutron Capture Spectroscopy in Quantitative Log Interpretation and Comparison with Conventional Interpretation: a Case Study
Authors: Kristianto Andy , Jong Yulianto , Moinard Laurent Louis 
Published: December 2008

The Elemental Capture Spectroscopy (ECS) log measures the concentrations of a number of elements (Al, Ba, Ca, Cl, Cu, Fe, Gd, H, K, Mg, Na, Ni, Si, S, and Ti) in the formation by neutron capture spectroscopy. These are used to compute mass fractions of certain components of the formation (clay, quartzfeldspar- micas, coal and carbonate). This log was recorded in a cored well of a large field in a deltaic environment with a view to checking the routine interpretation model of triplecombo logs.

The ECS log data were first validated against X-Ray Diffraction (XRD) measurements on 231 samples from a 166- meter cored interval.

A standard shaly-sand interpretation was performed using conventional logs (triple-combo) with the assumption that shale is made up entirely of clay minerals. This interpretation model had been used in over 600 wells in this field. XRD laboratory analysis confirms what can be expected: the socalled 100% shale point in fact consists of a lower clayfraction, 57% in our case. This is also consistent with the ECS log. An "adjusted interpretation" was performed, moving the wet-clay point to reflect the 57% clay fraction in shale. A third interpretation was performed using ECS clay fraction directly into the interpretation model. The question to be addressed was: "how are net-pay and hydrocarbon-porevolume values affected by the change of interpretation parameters and the addition of the ECS log to the logging suite?"

The main result is that there is no significant difference between net-pay and hydrocarbon-pore-volume values obtained from the standard interpretation (using conventional logs), the "adjusted interpretation" (moving the wet-clay point to reflect the XRD clay fraction in shale), and the new interpretation (integrating the ECS log in the interpretation model). Based on these findings, it was conclude (1) that there is no need to re-interpret logs in all the wells using the new wet-clay point and (2) that the ECS log should not be recorded on a routine basis in this field.

Introduction

The "X" field is the largest and the most productive field in TOTAL E&P INDONESIE located on the eastern limit of present Mahakam Delta. The main reservoir types are dominated with distributary channel and mouth bar facies. The 166 meters of cored interval, ECS log and conventional log were performed in "A" well on 2002.

Understanding the mineralogical composition of a formation is the fundamental of petrophysical analysis. In clastic reservoirs, shale fraction computation from log is critical, because its computation will be used for further computation, such as porosity and water saturation. Shale is a mixture of clay minerals, rock fragment, and fine-grained silicate minerals (silt) laid down in a low-energy environment (Adeyemo, 2005).

A standard petrophysical interpretation was performed over 600 wells in "X" field by using the assumption that shale made up entirely by clay minerals. Unfortunately, this assumption does not consistent with XRD laboratory analysis which tells that shale consists of lower clay fraction than the expected, which are 57% in our case. An "adjusted interpretation" was performed moving the wet-clay point to reflect the 57% clay fraction in shale.

The ECS logging is an unconventional logging which measures the proportion of elemental concentration and estimates the major matrix properties of a formation. The ECS log was consistent with XRD laboratory analysis from core data which tells that shale made up by 57% of clay minerals. A third interpretation was performed using ECS clay fraction directly into the interpretation model. The question to be addressed was: "how are net-pay and hydrocarbon-porevolume values affected by the change of interpretation parameters and the addition of the ECS log to the logging suite."