Projected increases in Algerian hydrocarbon production will come, in part, from more complex reservoirs. In Algeria, conventional quality reservoirs (>1 md) are interbedded with volumetrically significant, low permeability sandstones (<1 md)-the 'tight sand'. The challenge has been to develop a programme of work to establish reserves in this sub-millidarcy resource; on the whole, the results of this evaluation programme are positive.
The main tight sections in Hassi Messaoud are of Cambrian age. Grain-rimming chlorite cement within thin intervals locally inhibited later pervasive quartz cementation; the quartz is the cause of reservoir degradation in the bulk of the sandstone, with resulting permeabilities of <1 md, and often in the microdarcy range. Generally, the overlying Lower Devonian has a low density of faulting and fractures; open and closed fractures are observed in core, whilst mud losses suggest some fractures are conductive in the subsurface.
The permeability range in the tight sandstones extends below the resolution of conventional porosity/permeability measurement; determination has been improved through mercury-injection-derived permeabilities. Water saturations determined from core and logs suggest gas and oil may be present in the low-permeability rock. The presence of gas could not be confirmed by formation tester samples; thus, dynamic data is required from wells that were completed within the tight sandstone interval. The low leak-off coefficient suggests there is no connection to pervasive conductive fracture networks, although fractures intersected by the wellbores may contribute to flow. Also, large poro-elastic back stresses indicate that connections to high matrix or fracture permeability are unlikely. On balance, well-test analysis is a reliable indicator of hydrocarbon presence in very low permeability/porosity rocks that may recharge associated conventional layers following sufficient drawdown.
This paper highlights the main challenges of tight-sand well testing as well and classifies the problems that are specific to some reservoir in Algeria. Special emphasis will be given to the significant problem of horizontal well test interpretation that is typically encountered in the oilfield,, in particular, Algerian cases where the horizontal section of the drain passes through several different pressure zones. Attempts are made to identify and classify these problems, and recommendations are provided toward their resolution.
Additional horizontal well pressure-transient-analysis cases that have been executed and implemented with success in Algeria will also be highlighted, along with the benefits reached, disadvantages perceived, and conclusions.
Because of the complexity and variability of tight reservoirs in Algeria, the recommendations arrived at in this article present a general procedure for developing reservoir descriptions. In this paper, eight case studies of horizontal wells passing through tight reservoir are presented.