Title : Predicting drilling challenges and hazards due to subsurface pressure’s drifting
Abstract:
Objectives:
Drilling for oil and gas is an intricate process that required subsurface geoscience in coordination with engineering to complete the task of reaching the prospective trap. The open borehole formation - fracture pressures dictate the mud weight (MW) that controlling the drilling trip margin and the kick tolerance window. The pressure generated by Static (SMW) / dynamic (ECD) mud density against the wellbore wall formation pressures plays an essential role in drilling performance and safely. In this research , a brief description of the adequate pressure modeling and methods that support combating the anticipated flows, hard kicks, blowouts, lost circulations etc. before drilling.
Procedures:
In clastic sediments where sand, sandstone, clay and shale dominate the stratigraphic column, sedimentation process goes from deposition, compaction to lithification. This impacts the formations petrophysical properties (acoustic velocity, electric conductivity , density ) vertical changes with increasing depth. Analyzing the meandering of the interval seismic velocity (Pvi ) at the wildcat proposed location in association with sequence stratigraphy exhibit a great deal of the expected drilling challenges and their depth. Selecting the adequate modeling that concurs with the geological building blocks helps depth identifying and assess combating of the drilling challenging.
Results:
In general, the subsurface pressure profile can be divided hydrodynamically to four segments: a) normal static, b) compaction hydrodynamic c) pressure ramp ,and d) geopressured. In the Deepwater shallow zones (a-b) where sediments start the process of compaction, a hazard of shallow water flow (SWF) can occur due the differential pressure created by fluid expulsion. Drilling through the compaction zone (b) mud weight increases gradually to accommodate for the dehydration process . Deeper at the competent high stand shale seal ( c ) where compaction seized , a notable pressure ramp exists. This is usually associated with well flows, kicks, mud cuts and a notable mud weight increase to battle the abnormal excess pressure shift. The intermediate casing seat usually installs at this depth.
The sediments section (d) below the pressure ramp shows a succession of compartments in a cascade fashion form in the reservoir type formations. These compartments exhibit hydrostatic gradient and yield notable different excess pressure envelopes. It is referred to it as the geopressured sediment column. The presence of compartmentalized reservoirs charged with excess pressure and alternating with seals in this geopressured section makes open hole very vulnerable to flows, kicks, lost circulations and borehole instability.
Conclusion:
Before drilling, analyzing the pore pressure profile deduced by the carefully processed seismic velocity can be a great help in predicting possible drilling hazards. Introducing the geological building blocks in collaboration with the pore pressure modeling algorithms is a game changer. Most of the flows, kicks and blowouts, lost circulations take place when the drill bit crosses the lithological boundaries between shale (seal) and sand (reservoir), especially if geopressured pay zones present
