Abstract

This paper presents the results of propped fracturing operations conducted in the past 12 years in the Bach Ho (White Tiger) field offshore Vietnam. High temperatures (>275oF) and closure stress (>8,000 psi) combined with the fact that fracturing has to be performed from a vessel make the execution of fracturing treatments operationally difficult and challenging. More than 60 treatments have been performed in over 40 wells placing over 3 million lbm of proppant with a success rate greater than 85%. The wells targeted were both injector and producer wells. The post treatment results have been excellent with an average increase in Productivity Index (PI) of greater than 5 times.

This study discusses the properties of the various hydrocarbon-producing zones in the Bach Ho field. A general discussion of reservoir properties and damage mechanisms is included, which demonstrates the potential for stimulation and the associated benefits. The methodology adopted in the design, execution and evaluation of the fracturing treatments is discussed. Discussion on the rig-up of the fracturing vessel and the necessary modifications for efficient operation is also included.

A review of the treatments indicates that the majority of them are conducted at a flow-rate of 20–25 bbl/min (bpm), with a maximum proppant concentration in the range of 8–10 ppa (lbm of proppant added to 1 gal. of clean fluid) and proppant quantities of 60,000 to 80,000 lbm per treatment. High leak-off limits higher proppant concentrations and volumes. Higher rates are desirable but are limited due to limitation of equipment on the fracturing vessel.

Zones targeted are both the Oligocene (sandstone) and the naturally fractured Basement. Propped fracturing has been found to be very successful in the Oligocene whereas different types of treatments like propped fracturing, acid fracturing and acidizing have been tried in the Basement with limited success. Finally the lessons learned and the experiences gained in this field are shared.

Introduction

The Bach Ho field is located in the South China Sea, 120 kms south-east of the port of Vung-Tau in Vietnam. The field was originally discovered in 1974 but was placed in production only in 1986. Production is from the Miocene, Oligocene and the Basement formations flowing into a Floating Production Storage Offshore (FPSO) loading vessel. Average seawater depth is 165 ft. There are 215 wells in the White Tiger field which are mostly directionally drilled up to 70 degrees angle. There are also 3 horizontal wells.

The oil-bearing horizons in the White Tiger field are found in 3 main reservoirs:

  1. Miocene which is at depths up to 10,000 ft TVD

  2. Oligocene which is at depths up to 14,100 ft TVD

  3. Naturally fractured Basement which is up to 14,800 ft TVD

The primary target is the fractured granite basement which is typically oil saturated with a gas cap. The secondary target is the lower Miocene and Oligocene, which are interbedded sandstones saturated with oil. Some of the wells show initial production as high as 10,000 bopd.

The drilling practices and other damage mechanisms discussed in the next section restrict most of the wells from producing to their potential from reservoir engineering considerations. Hydraulic fracturing has been found to be the most effective technique to bypass skin-damage in the near-well bore region and create a high conductivity conduit for production enhancement. Fracturing was started in the Bach-Ho field as early as 19941. Target zone for fracturing has mostly been the Oligocene as it has reasonable recoverable reserves, does not have water zones nearby like the Miocene and is not naturally fractured like the basement. Some treatments have also been targeted in the basement but with limited success due to very high leak-off.

This paper discusses the design, execution and evaluation cycle of the project along with the problems encountered in the execution and the lessons learned.

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