Benchmarking LWD Sourceless Neutron Gamma Density Measurements in Southeast Asia
Authors: Kyi Ko Ko , Lwin Moet Moet , Maeso Carlos Jeronimo , Roberts Iwan Dewi Bowen 
Published: March 2015

Bulk density is a key petrophysical measurement that can be obtained from gamma-gamma density (GGD) and sourceless neutron-gamma density (SNGD) measurements. The unique SNGD measurement is new to the industry, having been introduced in 2012. A series of multi-functional LWD tools have been upgraded to include the option of acquiring density from conventional Cesium-137 (Cs-137) sourced gamma rays (GGD) and from electrically controlled pulsed-neutron generator (PNG) sourced gamma rays (SNGD). The two measurements are totally independent and can be acquired simultaneously. Multiple SNGD – GGD datasets have been compared from different fields in Southeast Asia to validate the SNGD measurement, providing confidence in the measurement in the study region. The extensive database includes the data from vertical to horizontal wells; different mud systems; limestone, sandstone and shale formations; and gas-, oil-, and water-bearing intervals. The results show excellent correlation between SNGD and GGD measurements. The average difference between the measurements is 0.001 g/cm³ over the whole dataset. This is well within the SNGD measurement accuracy specification of 0.025 g/cm³ for clean formations and 0.045 g/cm³ for shale.

The SNGD measurement has applications in all wells where there is a risk of losing bottom hole assembly (BHA) containing radioactive sources and in jurisdictions with tight nuclear regulations. There is strong interest in the measurement in the studied region, for several reasons: firstly, in development wells where variable depletion and shale instability are high risks; secondly, in exploration wells targeting deep zones with pore pressure ramps and very tight mud weight windows; thirdly, in long tortuous horizontal wells; and fourthly, wells risking total losses such as pinnacle carbonates. In addition, the multi-functional tool provides increased operational efficiency, higher rate of penetration (ROP) capability, significantly improved reliability throughout the system, and greater ease of maintenance. Replacing the Cs-137 source with a PNG significantly reduces the operational risks normally associated with the use of traditional LWD tools. By design, the PNG can be turned on only while pumping and only when several very restrictive safety conditions are fulfilled.

The study results justify placing confidence in the SNGD measurement in high-risk drilling conditions. To date, the sourceless neutron-gamma density has been utilized standalone in more than twelve high-risk wells in the region.

Introduction

Bulk density is a key measurement in the oilfield industry and is commonly used for lithology, porosity, and fluid characterization. Today in the industry, bulk density is predominantly obtained using a Cs-137 chemical source for gamma ray production. This is known as gamma-gamma density (GGD) and is commonly deployed on both wireline and LWD. The recent introduction of the pulsed-neutron generator (PNG) based sourceless neutron-gamma density (SNGD) measurement provides an alternative to the traditional GGD. The SNGD measurement is part of a comprehensive suite of measurements in a single short (26-ft) logging-while-drilling collar, as shown in Fig. 1. Enabling tight integration of these measurements, the PNG is positioned under the resistivity array. This electrically controlled source removes the need for a traditional Cs-137 chemical source for density measurements and, in addition, enables neutron porosity, sigma (macroscopic thermal neutron capture cross section), and elemental capture spectroscopy measurements. The PNG also represents a significant reduction in radiation risk, both for personnel and the environment (Weller et al. 2005). The focus of this paper is the density measurements.