INTRODUCTION:

Oil reserve is dependent on how much oil that can be recovered from reservoir. The recovery factor, ratio between recoverable oil to oil in place, is dependent on several factors. Some of those factors are driving mechanism, reservoir properties, fluid properties and so on.

In general, any oil field will go through several phases during its production lifetime. Those are natural flow phase where oil has its energy to flow to the surface, artificial lift phase where additional energy is introduced into producing wells to be able to lift the fluid to surface, pressure maintenance phase where additional energy is introduced into the field usually not directly into oil reservoir and the last one is enhanced oil recovery where rock/fluid properties are modified to reduce flow resistance or increase oil mobility.

Different methods of enhanced oil recovery have been developed in the industry. Those are steam flooding where steam/heat is introduced to reduce fluid viscosity, water flooding where water is injected into reservoir to sweep the oil, surfactant injection where chemical is injected into reservoir to reduce surface tension of rock and polymer flooding where polymer is injected into reservoir to increase sweep efficiency.     

Fig. 1  Comparison between water and polymer flooding

The common issue with water flooding project is early breakthrough between injector and producer wells. It happens where water flow through area with higher permeability directly to producer well and leaves large unswept area with significant oil remaining. This phenomenon is known as fingering. The illustration of fingering effect can be seen on Figure 1.

Generally, polymer injection is quite similar to water injection where water is injected into reservoir to sweep the oil from area with flow restrictions or low permeability. Polymer injection is expected to have uniform front to sweep the oil from reservoir avoid fingering effect.  

Fig. 2.  Vertical sweep

Pre-slug component is injected through injection well, usually polymer with high viscosity, to act as uniform front sweeping oil from reservoir. Main slug, usually polymer with lower viscosity than pre-slug, is injected into reservoir after a uniform front of pre-slug is established based on detailed analysis of rock and fluid properties. Surface parameters are recorded and samples are regularly taken to monitor sweep performance. Recorded data such as injected volume, tubing head pressure, fluid viscosity, fluid rate (including water and oil rate) are gathered and compiled to measure the polymer performance.

Polymer performance can be degraded due to environment effect, such as pressure, temperature and rock impurities. Fe²⁺ ion accelerates polymer degradation which reduces sweep efficiency in which different compound of polymer is required to decrease polymer degradation rate.

CASE STUDY:

In cooperation with our principal PT Enertech Indonesia, we worked on polymer flooding pilot project in Widuri field, CNOOC SES. This project started in Dec 2013 and ended on Dec 2015 and it involves one injection well and one producer well. Two stage injections were performed in injector well consists of profile control with pre-slug and main slug with post-slug. The first part is intended to minimize slug fingering effects which reduces the main slug effectiveness.

Detailed fluid analysis was performed to ensure that injected polymer is compatible with reservoir fluids. Impurities that would affect polymer performance were identified and solutions to manage them were developed. Once the proposed solutions were implemented, polymer injection was commenced with surface parameters were carefully monitored and recorded.  

Profile control injection was injected for one month along with pre-slug injection. Subsequently, main slug was injected for approximately 23 months into injector well. Apparent parameters related with each well were monitored and recorded at regular interval. One of the parameter, Pressure drop curves can be seen on Fig.1 below.

Fig.1 Pressure drop in injector well

The polymer was injected in injector well which was previously used as water flooding injector. The effect on surface parameter between water flooding and polymer flooding can be seen on

Fig. 2 below. 

Fig. 2, Injection result

It can be seen that steady increasing tubing head pressure with constant injected volume was achieved during polymer flooding, while maintain constant fluid viscosity. It was indirect indication that a better polymer sweep between injector and producer well was achieved compare to water flooding.

Fig. 3 shows the response on producer well. It required approximately one year to actually identified polymer injection effect in producer well. Even though slight decrease in fluid rate was seen, lower water cut yields additional volume of oil produced. 

Fig. 3 Production response

The result showed water cut reduction from 98% to 96% at constant fluid rate of 1,800 bfpd, which equivalent to oil rate increase of 36 bopd. Based on the result above, it can be safely concluded that pilot polymer project in Widuri is a success.

Due to administration issue, polymer injection was ceased on Dec 2015 although the initial result was encouraging. It could have expected that oil production would have been increased if polymer injection were continued.