Download to read offline
Formation damage introduction.pptx
- 1. FORMATION DAMAGE 1. Formationdamage can be defined as a part of the drilling process which impedes the ability of a hydrocarbon bearing rock to be able to flow to its full natural potential. 2. Whether or not the rock is damaged, can be ascertained by virtue of skin factor of the drilled rock, which if > 0, is an indication of damage. 3. Higher the value of skin factor, more is the extent of damage. Conversely if the skin value is negative, it means that the process of drilling has triggered the rock to flow more than its originally estimated recoverable reserve by primary technique. 4. Zero value of the skin factor is an indication that the rock is neither damaged nor triggered for higher flow by drilling process. 5. The first thought which immediately comes in most professionals’ mind while referring to formation damage is – The damage is due to mud!! 6. This is a major reason of not being able to eliminate/minimize formation damage to its full potential, as MID IS NOT THEONLY CAUSE!
- 2. FORMATION DAMAGE Havingsaid that, it is also a fact that incorrectly designed mud and improperly maintained properties of mud do cause fair amount of damage to the producing rock It is essential to understand and acknowledge that the very process of drilling is inherently damaging to the rock. Therefore until recently it was believed that it is not possible to drill the reservoir and NOT DAMAGE IT!! This myth has been busted and wells have been drilled with negative skin factor, an indicator that rock’s natural ability to flow has been enhanced by drilling fluid related aspects!! Let us remember - DAMAGE MINIMIZATION ATTEMPTS MAKE SENSE ONLY FOR RESERVOIR SECTIONS.
- 3. FORMATION DAMAGE Featureconducive to damage, that are extremely difficult to control are Naturally damaging rock. These are the types of rock which have very high permeability and porosity, leading to excess intake of fluid entering through the inter-connected and large pore spaces Composition of connate water and wettability features of rock are often capable of cause of damage, that cannot be completely overcome. Regression and/or abrupt ramp in pore-pressure can sometimes pose challenge in well design and mud properties selection to the extent that damage cannot be completely avoided Design optimization, sometimes calls for accepting minimum damage for the increased overall success of the project and adopt technology which negates the impact of drilling related damage. Filter-cake dissolving spacer technology, fracturing the rock to deeper horizontal reach, are some features
- 4. FORMATION DAMAGE Drillingpractices and drilling fluid design as well as management practices often entangle with each other to a large extent when it comes to damage. Therefore, a holistic approach of good oil field practices, is more preferred way of addressing the damage minimizing practices and the Dos and Don’ts described below are hence not being segregated into distinct two categories of drilling and drilling fluid. Do’s and Don’ts to minimize/eliminate formation damage are as below As far as possible, drill with just enough mud weight to keep reservoir content in its native place during drilling and the well-bore mechanically stable. Geo- mechanical modelling studies help achieve this. Do not drill at a rate faster than the overall ability of the drilling process to keep the hole free of cuttings. ECD monitoring is a viable technique to monitor this.
- 5. FORMATION DAMAGE Donot drill at a rate faster than the overall ability of the drilling process to keep the hole free of cuttings. ECD monitoring is a viable technique to monitor this. Focus on mud design to help build a compact and impermeable filter cake around open hole quickly to disallow filtrate invasion. If information is available on inter-granular pore space, then use the same wisely to design a mud with particles in such a way that they do not invade the pore- spaces and cause fluid migration along with filtrate. Mud weight more than optimum, will exert unnecessary extra pressure against the wall of open hole, resulting into filtrate invasion which will displace the reservoir content too far to activate the well during flowing back. Retain the native wettability of the rock by selecting the correct type of mud system to disallow damage of rock as a result of incompatibility between reservoir fluid and mud filtrate.
- 6. FORMATION DAMAGE Ifthe reservoir bearing open hole is to be cemented, then the slurry design should ensure that there is no mud / cement slurry lost during cement job. Software models help in achieving this. The rheological profile of the mud should be conducive to hole flushing without exerting unnecessary additional ECD. Ideally the LSRY ( 3RPM*2/6 RPM) should be equal to the size of the open hole from which the fluid is expected to flow back. While designing well clean-up programme avoid use of too many fancy stuff (surfactants /solvents/corrosion inhibitors, oxygen-scavanger) unless extremely essential question the logic of inclusion by assuming that default pill programme does not require any of these chemicals. Plain brine of correct weight should clean the fluffy part of filter cake and create pin holes in compact filter cake to facilitate flow of oil/gas through them which will then find their path out once the draw down is applied due to difference in formation -pressure and applied pressure to activate the rock.
- 7. FORMATION DAMAGE Mostimportant part of preventing formation damage is tom select a mud system after conducting core flood tests on various formulations in a reputed core lab. Selection of completion design also is an important aspect of preventing formation damage and completion engineer should be an integral team member to finalize the plan. Monitor dynamic Fluid loss rather than static API F/L and use a PPT apparatus with ceramic disc of size matching with average pore throat size of the reservoir rock. The PPT must be kept below 2.5 cc and tests carried out at BHCT and not on predefined API temp., which are good for static measurements. Formulate mud with fragile gels to negate the adverse impact of excess pressure on rock while initiating circulation after considerable gap. While selecting brine weight always remember to apply temp. correction factor to arrive at a realistic weight at bottom and know the corresponding weight at surface temp. The well bore clean up plan should be drawn with great caution to prevent excessive pressure getting exerted on well bore while various density fluids entering the annulus.
- 8. • Whole/partial mudloss into the producing rock is one of the biggest threats to formation damage and a fool-proof plan MUST be drawn to prevent loss of circulation as far as possible • While drilling through Limestone or fractured rock it is very difficult to prevent losses and ideally such formations should be drilled with floating mud cap technique or plain water (or brine of correct weight) to minimize damage. Such rocks are invariably acidized subsequently and use of water / brine of correct weight helps in ensuring that there is no incompatibility issue. • Do not tempt to drill at a rate faster than the ability of system to flush out cuttings no sooner they are generated. System efficiency can be improved by higher LSRV readings with fragile gels and optimized YP. • If situation warrants, flush the hole with low-hi vis pills without creating ay pressure fluctuation in circulatory system. Monitor ECD very carefully and never allow it to suddenly creep up. If required drilling can stop for a while and hole circulated to flush the cuttings and bring ECD in realistic range. FORMATION DAMAGE
- 9. • Do rememberthat with increase in depth there will be a nominal increase in ECD and this will not adversely affect the hole, but any abrupt rise in ECD must be addressed immediately and brought back in same range as prevailing before. • Last, but not the least – ALWAYS keep experienced and knowledgeable personnel on rig to protect the interest of the well. RESERVOIR DRILL-in Fluid For the last three to four decades there is an increased awareness level as well as willingness to address formation damage elimination / minimization by thoughtfully designing the mud system with focus on means of eliminating mud related damage during drilling as well as during well flowing. With scientific approach, a system has been devised which addresses FORMATION DAMAGE
- 10. • Filtrate invasionminimization • Quick formation of impermeable and thin, yet compact filter cake • Thoughtful selection of mud products. • Use of technical data to select suitable mud weight and not over emphasise only on well safety even if it comes at a cost of damaging the well/rock. • As far as possible, elimination of additives which have known potential to damage the rock by chemical/physic-chemical mechanism. • Knowing the type of rock and using the same as an input in selecting the composition and the type of mud system. • Selection of mud parameters which avoid/minimize formation damage. • All these factors led to evolution of a type of mud which is intended to be used specifically only for reservoir section of the well and such system has been distinguished by the other types with the name of “ Reservoir Drill-in fluid” system. FORMATION DAMAGE
- 11. • The primaryfunction of this system is to not only facilitate smooth drilling, but simultaneously attempt to protect the rock as much as realistically possible. One such essential feature of this system is that it is designed with focus on selecting the particles in mud of that size which does not intrude into the rock and make conscious attempt to maintain the size and concentration in as much the desired range as possible by continuous replenishment of sized particles in mud externally and compensate for loss of particles due to crushing action of bit while continuous circulation. At the same time, one has to select correct configuration and size of shaker screens to dispel finer particles and keep inert solids in a range which does not adverse the rheology of the system and yet build the much desired firm filter cake ( internal as well as external). • A lot of emphasise should be laid in correct use of solids control equipment to retain essential sized particles and only selectively discard harmful colloidal sized particles. This is being achieved by making arrangements of re-introducing the correct sized particles into main stream from shaker discard. FORMATION DAMAGE
- 12. Selection of variousadditives in RDF mud • The focus, again is on eliminating the additives which are generously used in drilling upper sections, like PAC, Asphalts etc to name few. Instead of them bio- degradable starch as well as bio-degradable viscosifiers are used to build the skeleton of mud which is then converted in a RDF by administering correct sized particles of calcium carbonate and avoiding use of barite and instead achieve desired weight either by high density brine or by adding higher dose of calcium carbonate or a combination of both. The native wettability of the rock is ascerta9ined and as far as possible simpler version of water-based mud is preferred over other types, unless severe shale instability issues warrant otherwise. • It is imperative to accept that the process of drilling will invariably damage the rock and efforts are made to minimize the impact as much as possible. One can aim for zero damage but that is not always achievable. The wisdom is to live with minimum damage and yet drill the rock safely and timely. FORMATION DAMAGE
- 13. When to useRDF: The decision to use RDF must be taken with all factors being given due weightage and selecting the system as well as components and properties which not only minimize damage to the rock, but also facilitates safe and timely drilling. Wherever feasible, core flood tests should be conducted in lab with samples of probable mud type and system to find out which of them provide highest permeability regained figure. Out of the top few such formulations the ease of drilling and completion design chosen for the well have to be reviewed. It may not always be possible to select a system which is zero damaging but the one which facilitates hassle free drilling and completion as well as damages the rock to the least, is often the chosen one. With advancement in drilling fluids technology now it is possible on many / all occasions to use solids free design of RDF and yet get the benefit of drilling and completion with minimum complication. Solids-free RDF in reservoir actually means that whatever solids are there in system,, they are purposely retained to facilitate damage minimization and other non-essential additives (Barite or other heavy density additive which has potential to damage the rock). FORMATION DAMAGE
- 14. CHANGING OVER TORDF AND MAINTAINING RDF • Once a consensus has been arrived by team to drill the reservoir with a particular type of mud, it is imperative that the mud of previous system is completely replaced with RDF correctly formulated, including but not restricted to hole volume, surface volume. Before resuming reservoir drilling it is a good practice to build at least twice the open hole volume of the section planned for drilling to account for additional volume requirement, failing which often there is a temptation to blend the previous section mud. This can defeat the purpose and should be refrained from. Monitor particle size of the active mud with the help of particle size analyser to be made available on rig with qualified person to operate the same periodically and help mud engineer to decide his tour dose of essentially solids of correct size. This aspect is often overlooked and is a cause of not being able to eliminate rock damage completely. FORMATION DAMAGE
- 15. SUMMARY • It isa bit unrealistic to conclude that use of RDF will eliminate formation damage but certainly, with right planning and correct execution the extent of damage can be brought with- in manageable limits. The concept still needs enormous support from the entire team for optimized results and success/failure ca not be entirely executed to any single individual. • USE OF RDF SHOULD PROVIDE FLEXIBILITY TO ELONGATE THE LIFE OF THE WELL AT SUSTAINABLE FLOW RATE. EARLY SPURT AND UNSUSTAINABLE RESULTS REFLECT PARTIAL SUCCESS ONLY. FORMATION DAMAGE
- 16. SOLIDS FREE NON-DAMAGINGDRILLING AND COMPLETION FLUIDS • Industry has now by and large realized the importance of solids-free drilling as well as completion fluids for various advantages they offer to drill and complete with. • Without going in those details and focussing only on damage minimization aspects of such fluids, these fluids offer • Minimized fines migration • Formation-friendly filtrate ( in some specific type of brine) to enhance the flowability of the producing rock by virtue of chemical reaction with connate water of the rock and converting it to a flow-inducing fluid • Minimized solids in filter-cake, resulting into its easy lifting feature • Complete avoidance of incompatibility between drilling and completion fluid by using same base fluid in drilling and completion fluid ( brine) FORMATION DAMAGE
- 17. As a resultof the capability of some heavy weight brine/s to eliminate use of weighing material completely, such brines offer huge advantage in HTHP wells as they eliminate fines migration( and thus control intractable formation damage), apart from offering host of other drilling related advantages – reduced ECD, reducing flow-check time (as they attain thermal equilibrium very quickly) etc. They impart the flexibility of loading higher concentration of proppants and yet transport the entire fluid to fractured zone by providing excellent solubility in presence of VES( Visco-elastic surfactants) The challenge in such fluids is to keep the fluid-loss ALAP, because the solids required to retain/disallow flow of water from mud due to pressure application, are either absent or insufficient. FORMATION DAMAGE
- 18.
- 19.
- 20.
- 21.