The application of sedimentology in the characterization of borehole gravel packs and screen slot sizes for the Nanka and Ajali formation aquifers, Anambra Basin, southeastern Nigeria

Abstract

The drilling of a borehole into a poorly cemented and unstable reservoir/aquifer system like the Ajali and the Nanka Sands and the withdrawal of its fluids trigger movement, because unconsolidated grains and matrix materials are dislodged and entrained in the produced fluid (water, gas or oil). The undesirability of pumping any fluid together
with sand and matrix material is reflected in the dangers posed to the equipment by way of ; abrasion of downhole pumps and accessories, casing pipes and surface equipment on one hand; and the loss of production, compaction of reservoir and handling/disposal problems on the other hand. Produced sand thus must be stopped, while allowing formation fluid to pass into the borehole. Two measures of sand control for the Ajali and Nanka fluid production are discussed herewith; a screen placed at the reservoir interval and a gravel pack in the screen-hole annulus. Gravel pack character and screen slots design are directly dependent on the size of formation/aquifer sand size. Relevant equations were used to compute the sizes of gravel pack material and screen slot sizes for twenty boreholes; eleven (11) that penetrated the Ajali and nine (9) that penetrated the Nanka Sands, (the two most prolific and most exploited aquifers within the Anambra basin). Results show that gravel pack sizes for the Ajali Sands range from 1.42mm, through 2.13, 3.37 to 5.06mm, indicating an average gravel diameter size range of 2.13 to 5.06mm. For the Nanka Sands, the gravel pack values range from 0.57mm, through 0.75, 3.17 to 4.73mm indicating an average range of 0.57 to 4.75mm. Uniformity coefficients (Uc) for the Ajali and Nanka Sands are 0.6 and 0.32 respectively, indicating perfect or nearly so, but it is broad for most individual boreholes. Under broad uniformity conditions, recomputed screen slot widths are 0.33 and 0.18mm for the Ajali and Nanka Sands respectively. In commercial terms, these figures generally translate into ‘24inches gage’ and ‘7inches gage’ screen slot sizes for the Ajali Sands and ’13 inches gage’ for the Nanka Sands.

KEY WORDS: Ajali and Nanka Sands, Sand Control, Gravel Pack and Screen Slot Sizes