We know that when we drill formations, there a column od mud drilling in wellbore that satisfies required pressure for preventing flow of formation fluids into wellbore. If we use a low weight mud in a high pressure formation, a flowing path occures from formation into wellbore. In impermeable rock such as anhydrite or salt for example in Gachsaran Formation (Iran Oil Fields), the brine folw is occured. These are impermeale rocks and as far as i know there isnt any flow path in these type of rock, But how can brine flow? What is the brine production process?
The bedding plane would be only passage for naturally moving fluid from formation to borehole. But if you mean artificially the acidizing and some additives.
The only way that seems reasonable to me is a conducting fracture network. From anhydrite or salt you would not expect flow from the matrix since this is probably
I would enrich Andreas's answer by adding that it is rather common to have naturally fractured thin brittle dolomite stringers interbedded in most anhydrite or salt sequences, and that those permeable stringers (a few inches to a few feet thick) are usually extremely overpressured, with mud density equivalent gradient above 2 (water = 1), i.e. more than twice the normal hydrostatic gradient.
Any fluid (brine, gas, oil) flow from the formation to the borehole while drilling a salt/anhydrite sequence with relatively low mud density would come from those overpressured dolomite stringers.
I agree with you and thank you. But Roberto said something about dolomite which are in salt or anhydrite formations. Let me ask clearer. In Iranian oil field, there is a formation named Gachsaran which is evaporate lithological rock. It consists of anhydrite, salt and marl. This formation is divided to 7 members that are named from 7 to 1. Gachsaran 6-2 are high pressure so we use 13 3/8" casing point at top of Gs6 and 9 5/8" casing point at Gs1 or cap rock of Asmari reservoir formation. When we drill section 12 3/8", use heavy weight mud (138 to 140 pcf). If the weight decreases there is flow from formation that is anhydrite or salt or marl. This flow some themes comes up and make a blow out.
Is the Roberto's answer true for mechanism of this situation?
Thank you
Alireza, As you said "In impermeable rock such as anhydrite or salt for example ........ the brine folw is occured. " Well, by definition, "impermeable" means that there is NO fluid flow in the rock matrix itself, so the only possible flow mechanism is vug or fracture systems that are connected. And they have to be connected. Now it is more then possible that the fracture system (including vugs) can be at either high or low pressure, and either way your mud system is going to have difficulty in dealing with it. But the flow - by definition of "impermeable" - is not originating from the rock matrix. It can only be the secondary porosity systems represented by the diagenesis of the dolomite and anhydrite sequences. The marls in between are probably acting more like a shale than anything else. Salt fractures are possible, but I would personally put it firmly with the secondary porosity systems. Hence Roberto's answer is probably correct - although the fractures can be in any of the dolomites or anhydrides.
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