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Hole instability in Nile Delta area, specially in the Middle and East leases is one of the main drilling problems encountered in most of the wells drilled either by Petrobel or Nidoco. Shale swelling, caving, tight holes, and consequently loss of circulation problems were experienced and overcame using several techniques. This paper is focused on the analysis of the main drilling hazards related to shale instability including analysis of pore pressure data obtained from sonic, sigma, and RFT logs and prediction of pore pressure expected in the forecasted wells. It discusses the different mechanisms of Mile Delta shale instability. Field observations and results of X-Ray diffraction analysis of shale samples collected from the area wells led to a significant improvement in shale stability by using potassium polymer mud instead of sodium chloride lignosulphonate mud. Also tight hole and differential pipe sticking possibilities while drilling through low pressure porous sands were reduced by using ultra fine cellulosic materials.
Conclusion and recommendations for drilling performance optimization are included in this paper.
Abu Madi gas field located in the northern part of Nile Delta, discovered by IEOC in 1967 and completed and developed by Petrobel, fig.(1).
Maintaining a stable borehole is one of the main objectives while drilling gas wells in Nile Delta area. Hole instability in the area takes several forms, soft ductile bentonitic shale squeezed into the hole, hard brittle geopressurized shales that spall under stresses, and caving due to swelling pressure, with consequent hole enlargement and bridging. This results-in disasters such as stuck pipe and sidetracked hole.
Also, loss of circulation into high porosity unconsolidated Sands of Mit Ghamr and through depleted sands of Abu Madi formations, with its consequent related drilling and mud problems, was encountered. These problems adversely influence drilling efficiency and cost. Analysis of 33 wells drilled in the area showed significant variation according to mud type, formation pressure and drilling practices.
The followings are the main challenges to drill a trouble free well in Nile Delta: 1- Optimum mud weight to drill through Kafr El Sheikh shales based on its actual pore pressure. 2- Optimum mud type to alleviate the hole instability problems experienced while drilling Kafr El Sheikh, and the lower formations. 3- Optimum casing design for exploratory and development wells. 4- Optimum Nile Delta drilling practices.
One of the practical and reliable procedures for detecting and investigating hole instability problems is, to determine the actual formation pore pressure anomalies within Kafr El Sheikh and. Sidi Salem shales, to identify the clay minerals present in the problematic shale, to select the optimum mud system and to evaluate the area drilling practices.
I - ORIGIN OF OVER PRESSURE according to Fertle and others, origin of overpressure can be due to several factors. In Nile Delta area, the over pressure encountered through Sidi Salem and Kafr El Sheikh is mainly due to overburden and tectonic effects.
SIDI SALEM FORMATION
Overburden effect; Sidi Salem formation deposited after the pronounced flexure affected Pre-Miocene formations extending E-W across the mid-Delta area resulted Delta basin shown in figure 3 during the Middle Miocene age in deep marine environment (outer shelf). The formation is composed of thick body ( 700m) of shale containing few sand levels near its top. As the main composition is shale, the fluid remained trapped in the shale and as the zone exposed to the subsequent over burden loads (+3400 m) the pore pressure built up above the hydrostatic value. Tectonic effect; The high rate of subsidence of the basin while sedimentation of the overlaying deltaic sediments of Qwasem formation created differential compaction gravitational faults, in addition to some tectonic movements during late Messenian which created some tectonic stresses, faulting and diaper of Sidi Salem shale through Qwasem and Abu Madi formation.
DGE plans to drill wells in Pliocene reservoirs which are represented by Kafr El Sheikh (KES) and El Wastani formations of Nile Delta (geographical location Fig-1). Fig-2 shows these aforesaid formations and relevant mega sequences on a seismic section. Kafr El Sheikh is subdivided into three Mega-Sequences of this massive marine transgressive-regressive series of sequences and the sequence boundaries correlate with changes in the relative sea level and consistent with Global Eustatic Curve Changes (El Haq et al., 1989). The thickness of these formations increase toward the north. Kafr el Sheikh Unit rests conformably over the Abu Madi Formation and underlies the El Wastani Formation. It mainly consists of shales and clays which are generally intercalated with fine sand beds. El Wastani Formation consists of thick sand beds interbedded with a thin clay beds. The depositional environment of KES rocks range from inner to outer neritic with fluvio-marine conditions, especially in the southern mid-delta area. Onshore Nile Delta Pliocene age /KES formation gas reservoirs are dominated by slope-channel systems. Also, it is worth mentioning that the fluid content within KES FM is usually dry gas with in situ biogenic source to wet gas on the lower part from deeper thermal source rock.
Objectives: DGE recently carried out the prospectivity analyses of its Pliocene portfolio (mainly Kafr El Sheikh Formation) to identify the well locations and meet the business goals. The key objectives of the study were two-fold:
DGE have been drilling Pliocene sands since many years but only with a mixed rate of success. The study focused to identify the causes of well failures and define the way forward to improve the success rate. It is worth highlighting that until 2010, most of the wells were chosen for drilling mainly based on the negative high amplitudes (Normal SEG polarity) on full stack seismic data.
Abstract Exploration drilling in the Nile Delta has dramatically risen over the past 3 years, jumping from 5-10 wells/year up to 20-25 wells/year. Much of this recent drilling has been targeting the highly successful Pliocene play trend and the 2008 industry success ratio approached a phenomenal 90%. Thick sequences of Pliocene deep marine sediments have been successfully drilled and led to several offshore gas discoveries. No great exploration interest had been directed towards these good reservoirs in the onshore areas of the Nile Delta. Nevertheless, Dana Gas has been one of the most active and successful operators for this exploration target in its West El Manzala and West El Qantara Concessions. The gas sands of the Pliocene reservoirs are characterized by low velocities and densities compared with the surrounding shales and, subsequently, the top and base of the reservoirs have a "bright?? seismic amplitude response. Additionally, the gas charged Pliocene reservoirs are often associated to acoustic impedance anomalies presence. The key challenge in the Pliocene reservoirs exploration is to understand the depositional environment with respect to the amplitude and acoustic impedance geometry from 3D seismic data through the cross-correlation between seismic facies character and the depositional pattern and environment. The main features and variations recognized in the reservoirs drilled to date in West El Manzala and West El Qantara Concessions show a slow activity in the main basin evolution stage that seems to be not affected by high turbidity current flow or significant slope channeling. The early stage broad and straight channel system valley was cut and filled by younger channelized systems. Minor incisions of channel cuts are considered to be a key feature in defining channel fill development and sand distribution.
Abstract Geochemical studies of Parh Limestone, through major and trace elementanalyses, were made to evaluate its depositional environment. Parh Limestonerepresents Upper Cretaceous period in the Kirthar and Sulaiman provinces ofLower Indus Basin. The Parh Limestone of Turonain-Santonian age is well exposedin NNWSSE trending Pab Range of Balochistan, which merges into Kirthar FoldBelt northwards. The Indian Plate motion, sea-level fluctuations and volcanismwere the main controlling factors responsible for the distribution of elementsduring the deposition of Parh Limestone. During the cycle of deposition of ParhLimestone the distribution of Mn and terrigenous material (clay) revealed HighStand System at the beginning (Turonian, 91m.y.), which latter turned into LowStand System and finally terminated as Transgressive System with a very broadshelf environment (Maximum Flooding Surface). The Al/Mn+Fe+Al ratio of the ParhLimestone revealed slow rate of drifting initially and high rate at latterstages due to high rate of sea-floor spreading. The Ca/Mg, Ca/Fe, Mg/Fe ratiossuggested hemipelagic environment of deposition. The contents of Ba, Zn and Cowere higher than the average abundance in the limestone reflecting partialinfluence of igneous activity at the time of deposition. The composition of anigneous sill, at the base of Parh Limestone demonstrates oceanfloor tholeiites, affiliated with Mid Oceanic Ridge Basalts (MORB). Introduction Present study emphasizes to evaluate different parameters of depositionalenvironment of Parh Limestone on the basis of major and trace elementgeochemistry. Tectonically induced changes in sedimentation patterns, volcanismand other factors within the carbonate system are also discussed. The study andadjoining areas are underlain by sedimentary sequences ranging from Jurassic toOligocene (Table 1). The Middle and Upper Cretaceous sedimentary rocks (Goru, Parh, Moghal Kot, Fort Munro, and Pab Sandstone) are well exposed in the PabRange (Fig. 1) while in the SW mélange zone, exotic blocks of Loralai(Jurassic), Parh (Cretaceous), Jhakar (Eocene) crop out along with ophioliticmélange .