Multi-Scale Block Structure of Petroleum Pools
A multi-scale block structure of natural gas pools made up of two fundamentally different block types has been revealed.
1. Large-scale blocks (LSB). LSB were revealed mainly by seismic data. The contemporary methods of development design take into consideration only LSB. Usually each LSB is considered to be a separate pool hydrodynamically governed by Darcy’s Law.
2. Small-scale blocks (SSB). In the majority of cases SSB cannot be revealed by seismic data due to restrictions of seismic resolution. In most cases hydrodynamic connection between SSBs exists but inter-block crossflows are evident only under pressure gradients higher than some initial pressure gradient (IPG).
The multi-scale block pool structure is the most apparent with gas pool development under depletion drive.
We developed an SSB isolation algorithm within gas pools based on the following relationships:
1. SSBs are divided by two types of barriers. The barriers differ by orientation relative to pool bedding: transverse-type 1 and parallel-type 2. The barriers are composed of low permeability rocks usually containing gas and residual water. Gas filtration through most of the barriers occurs only under pressure gradients higher than IPG; the IPG values can reach several MPa/m. The extents of the revealed type 1 barriers reach only several meters. They are located in neotectonic zones. These zones can be localized by satellite imaging interpretation.
2. Usually both physicochemical fluid properties and gas-water contact elevations differ significantly within various SSB of the same pool.
3. After completion of gas extraction, reservoir pressures within the extraction zones remain lower than in SSBs with non producing wells.
A new block filtration gas pool model (BFM) is demonstrated by both theory and experiments. The analysis of the development results for 50 pools has shown that BFM more adequately describes filtration processes within a real pool compared with hydrodynamically coupled models which consider stratified or aerial pool heterogeneity.
Multi-scale block structure also occurs in oil pools.
Revision of natural pool structure with consideration for the barriers between SSB as well as selection of optimal development systems for each block enables to boost oil and gas recovery by 10-15% relative to initial recoverable reserves as well as virtually doubling recoverable condensate reserves.
AAPG Search and Discovery Article #90090©2009 AAPG Annual Convention and Exhibition, Denver, Colorado, June 7-10, 2009