Oil & Gas - Maximising Recovery Programme (formerly SHARP) IOR Views

Introduction

BP’s research studies indicate that injecting low salinity brines (<4000 ppm) can significantly improve the benefits of water flooding (1) and BP has reported that the LoSAL™ EOR process has the potential to add ~1 billion barrels to BP’s reserves worldwide over the next 20 years. This article presents results from laboratory studies and reservoir trials of the LoSAL™ EOR process.

Corefloods

A number of full reservoir condition core floods have been undertaken using live fluids, at representative reservoir stress.  The results of these are summarised in Table 1. To date all clastic reservoirs have shown a benefit in both secondary (low salinity injection from initial water saturation) and tertiary (low salinity injected after high salinity water) waterfloods.  The benefits of LoSAL™ range from an 8% to a 39% increase in the amount of oil recovered(2).

Reservoir	So (Initial)	Sor (High Salinity)	Sor (Low Salinity)	% Oil Recovered (High Salinity)	% Oil Recovered (Low Salinity)	% Increase in Oil Production over High Salinity
Reservoir A	0.82	0.33	0.25	60	70	16
Reservoir B	0.85	0.32	0.25	63	71	14
Reservoir B	0.80	0.21	0.14	74	82	11
Reservoir C	0.75	0.18		76		-
Reservoir C	0.77		0.14		82	8
Reservoir D	0.81	0.25		70		-
Reservoir D	0.79		0.13		84	21
Reservoir E	0.85	0.33		61		-
Reservoir E	0.81		0.12		85	39

Table 1: Summary of Core Flood Results

Figure 1 shows plots of results from the core floods.  On the left is cumulative oil recovery (as a fraction of pore volume) versus cumulative water injection (again as a fraction of pore volume) for a series of three primary floods.  The injection water used ranges from 100% formation water to a mixture with only 5% of the salinity of  formation water. Breakthrough occurs significantly later with the lowest salinity flood.   On the right is a similar plot for a tertiary low salinity flood which follows a significant period of  high salinity flooding; again there is a significant benefit from the low salinity flood.

Figure 1: Core Flood Results for Secondary (left) and Tertiary (right) Flooding

The differences in the habitat of residual oil can be clearly seen in the micro-visualisations shown in Figure 2.  A single plug sample was prepared to Swi and cut into two, ensuring that samples were identical.  The thin section slides show that there is significantly less oil remaining after the low salinity waterflood compared with the high salinity waterflood, and that the habitat of the remaining oil post high and low salinity waterfloods is different.  (Note in these micro-visualisations blue represents oil.)

Figure 2: Remaining Oil in Identical Core Samples Following High Salinity (upper) and Low Salinity (lower) Waterfloods (oil shown in blue)

Studies investigating the mechanisms behind the recovery of the additional oil are due to be published later in 2006 (3). However, the role of multiple ion exchange between injection fluid and rock surface has been found to be the main mechanism for increased oil recovery.

Logging Test

Repeat logging was used to quantify the low salinity effect in the near wellbore region of an oil reservoir.  Low injection rates were used to represent bulk reservoir rates, and the test was designed to ensure no cross flow occurred between reservoir horizons.  Remaining oil saturation to high salinity brine was deduced from a pulse neutron log using high and intermediate salinity brines.  Remaining oil saturation after low salinity brine injection was determined after injecting low and high salinity brines.  Figure 3 shows the increase in oil production which was attributed to low salinity injection.  This represents 20-50% additional oil recovered (4).

Figure 3: Logs Comparing Residual Oil Saturation After High and Low Salinity Waterflooding

Single Well Chemical Tracer Test
A number of single well tracer tests have been performed.  These tests are on a single well basis but measure average remaining oil saturation around 15 to 20 feet into the wellbore, over the open interval.  These have all shown increased oil recovery attributable to low salinity injection, and a significant decrease in residual oil saturation (5).  Figure 4 shows an example where the residual oil saturation has been reduced from 21% to 13%.

Figure 4: Results of Single Well Chemical Tracer Tests

In addition, several so far unpublished tests have been completed, including investigations into the impact of low salinity slugs.  All tests to date have shown incremental recoveries with low salinity injection.

Reservoir Scale

There is evidence from production data for a secondary waterflood performed in one of BP’s Alaskan fields which shows increased recovery in low salinity waterflood patterns, compared with patterns under high salinity waterflood.  Patterns selected for comparison were from similar areas of the reservoir, to ensure that oil and rock properties were as similar as possible. 

However, tertiary injection of low salinity brines remains, at this time, unproven at the reservoir scale.  Tertiary core floods and single well tests show that low salinity water flooding produces incremental oil at this scale and a reservoir trial is planned.

A number of other projects are also in planning within BP.

The VIP reservoir simulator has been modified to add an option to model the effect of low salinity water flooding at the reservoir scale.  The methodology behind this will be published in September 2006 (6).

References

1. Morrow, N.R., G. Tang, M. Valat, and X. Xie, 1998, “Prospects of Improved Oil Recovery Related to Wettability and Brine Composition” J. Pet. Sci. Eng., vol. 20, June, pp. 267-276.
2. Webb, K.J. Black, C.J.J Edmonds, I J. “Low Salinity Waterflooding- The role of Reservoir Condition Corefloods” Presented at the 2005 EAGE conference in Budapest, Hungary.
3. Lager, A; Webb K.J., Black C J J  “Low Salinity Oil Recovery- An Experimental Investigation” To be presented at the 2006 Society of Core Analysts 2006 Meeting in Trondheim, Norway.
4. Webb, K.J., Black, C.J.J., Al-Ajeel, H, 2004 “Low Salinity Oil Recovery- Log Inject Log” SPE89379.
5. McGuire, P.L, Chatham, J.R, Paskvan, F.K, Sommer, D.M., Carini, F.H. “Low Salinity Oil Recovery: An Exciting New EOR Opportunity for Alaska’s North Slope” SPE93093 Presented at the 2005 SPE Western Regional Meeting.
6. Jerauld, G. R. Lin, C.Y. Webb, K.J. Seccombe, J C. “Modelling Low Salinity Waterflooding” SPE 102239 To be presented at the 2006 Annual SPE Technical and Exhibition Conference in San Antonio, Texas.