Petrophysical interpretation and fluid substitution modelling of the upper shallow marine sandstone reservoirs in the Bredasdorp Basin, offshore South Africa

Abstract

The fluid substitution method is used for predicting elastic properties of reservoir rocks and their dependence on pore fluid and porosity. This method makes it possible to predict changes in elastic response of a rock saturation with different fluids. This study focused on the Upper Shallow Marine sandstone reservoirs of five selected wells (MM1, MM2, MM3, MM4, and MM5) in the Bredasdorp Basin, offshore South Africa. The integration of petrophysics and rock physics (Gassmann fluid substitution) was applied to the upper shallow marine sandstone reservoirs for reservoir characterisation. The objective of the study was to calculate the volume of clay, porosity, water saturation, permeability, and hydrocarbon saturation, and the application of the Gassmann fluid substitution modelling to determine the effect of different pore fluids (brine, oil, and gas) on acoustic properties (compressional velocity, shear velocity, and density) using rock frame properties. The results showed average effective porosity ranging from 8.7% to 16.6%, indicating a fair to good reservoir quality. The average volume of clay, water saturation, and permeability values ranged from 8.6% to 22.3%, 18.9% to 41.6%, and 0.096-151.8 mD, respectively. The distribution of the petrophysical properties across the field was clearly defined with MM2 and MM3 revealing good porosity and MM1, MM4, and MM5 revealing fair porosity. Well MM4 revealed poor permeability, while MM3 revealed good permeability. The fluid substitution affected rock property significantly. The primary velocity, Vp, slightly decreased when brine was substituted with gas in wells MM1, MM2, MM3, and MM4. The shear velocity, Vs, remained unaffected in all the wells. This study demonstrated how integration of petrophysics and fluid substitution can help to understand the behaviour of rock properties in response to fluid saturation changes in the Bredasdorp Basin. The integration of these two disciplines increases the obtained results' quality and reliability.

Determination of Reservoir Flow Units from Core Data: A Case Study of the Lower Cretaceous Sandstone Reservoirs, Western Bredasdorp Basin Offshore in South Africa

“…However, there is a paucity of published work on the sandstone reservoirs in the western Bredasdorp Basin. Some of published work on reservoir characterization and formation evaluation of the western Bredasdorp Basin includes, among others, those of Ojongokpoko (2006), Hussien (2014), Acho (2015), Maseko (2016), and Magoba and Opuwari (2017).…”

Petrophysical core-based zonation of OW oilfield in the Bredasdorp Basin South Africa

“…To determine the rock type in a reservoir, it is essential to understand the primary rock properties, such as the mineralogy, texture, grain packing, and other parameters because they influence the petrophysical properties of rocks 10 , 12 , 13 , 17 , 41 , 42 . Lucia's Petrophysical rock classification is a technique used to relate pore size distribution to rock fabric with laboratory measurements of porosity and permeability 43 .…”

“… 8 – 13 . An integrated petrophysical reservoir characterization method that involves core, well logs, and sedimentology helps to improve reservoir description by investigating various rock types that ultimately leads to an enhanced oil recovery 14 – 17 . Several studies 18 – 23 have reported that reservoir quality is controlled by the pore geometry, which determines fluid movements.…”

Petrophysical Core-Based Zonation of An Oil Field In The Bredasdorp Basin South Africa

“…To determine the rock type in a reservoir, it is essential to understand the primary rock properties, such as the mineralogy, texture, grain packing, and other parameters because they in uence the petrophysical properties of rocks 10,12,13,17,41,42 . Lucia's Petrophysical rock classi cation is a technique used to relate pore size distribution to rock fabric with laboratory measurements of porosity and permeability 43 .…”

“…Many authors successfully apply core plug analysis results of porosity and permeability in carbonate and classic environments to quantify the reservoir ow and storage character and determine reservoir quality, e.g., [8][9][10][11][12][13] . An integrated petrophysical reservoir characterization method that involves core, well logs, and sedimentology helps to improve reservoir description by investigating various rock types that ultimately leads to an enhanced oil recovery [14][15][16][17] . Several studies [18][19][20][21][22][23] have reported that reservoir quality is controlled by the pore geometry, which determines uid movements.…”