Characterization of Heterogeneous Carbonate Reservoir with Advanced LWD Technology: Case Study of LHX-1 Oilfield, South China Sea
Authors: Bo Liu , Li Huigeng , Ju Song , Gong Hong Jie , Donghong Luo , Yiming Wu , Yongjiang Zhang 
Published: May 2016

Abstract
Integrated with core data, geological data and logging while drilling (LWD) borehole resistivity image and density-neutron logs, this paper demonstrated a field case study to characterize the heterogeneity of LHX-1 coral reef reservoir in South China Sea; which includes geological facies, lithology, fractures, secondary pore system etc. This reservoir is featured with hyper heterogeneity resulted by overlapped reefs, which have created vertical and lateral variation.
High resolution LWD resistivity image logs were selected to address specific challenges and unknown:appraisal well with limited data in place;uncertainty regarding controlling factors on fracture and secondary pore system;structural uncertainties including reservoir buried depth and thickness; anduncertain controlling factors affecting original productivity and future production decline.
The use of the integrated dataset was successful in delivering a full characterization of the reservoir heterogeneity: Fracture distribution was recognized and fairly understood in planar view and analyzed from qualitative and quantitative perspectives, secondary porosity was estimated and analyzed, and the controlling factors on productivity were clarified. The integrated analysis processes and expertise gained in this case study could be referred in other field development projects, not limited in carbonate reservoirs, but also in other heterogeneous reservoir such as turbidite or tight sand.

Introduction
Borehole resistivity image logs are widely used during the exploration of carbonate oilfield; generally it's combined with conventional logs or cores. Harthy etc. 2010 built a methodology to characterize, quantify and predict the reservoir heterogeneity across the field by utilizing wireline borehole image and nuclear logs. However, during the oilfield development stage, there are very few published papers to characterize the carbonate reservoir heterogeneity, particularly the coral reef reservoir. Therefore, it is very important to fully understand the reservoir variation in rock properties and qualities during the drilling process to optimize the decision-making, and use the integrated analysis to guide the future reservoir monitoring and management.
The LHX-1 oilfield is located in the central uplift of Pearl River Mouth Basin in South China Sea, and it covers approximately 14.4km2. The reservoir target is developed at platform margin reef, which is bounded by a major fault. The elongated margin reef trends from northwest to southeast, the reef is steeper toward the reef front and gentler toward the reef back.
Its' different structural features enable the reef to be divided into reef core, which developed coral skeleton limestone; backreef slope, which developed laminated algal limestone and micrite; and backreef platform which developed massive algal limestone. The first exploration well 1 was drilled at structural crest in 1987 and it proved potential oil production for this oilfield. Drill stream test (DST) in the first exploration well revealed the productivity was 81.1m3/d before acidification and 282.6m3/d after acidification. Core data showed the average porosity at 14.3% and logging data showed the average porosity at 14.0% in the reef core. In the backreef platform, core average porosity was 24.4% and log average porosity was 20.5%.