الگوريتم روش خط جريان براي شبيه سازي جريان تك‌فاز و دو‌فاز غير‌قابل تراكم در مخازن هيدروكربني دو‌بعدي با شبكه‌بندي باسازمان

A STREAMLINE ALGORITHM FOR SOLVING INCOMPRESSIBLE ONE AND TWO-PHASE FLOW PROBLEMS IN TWO DIMENSIONAL RESERVOIRS USING STRUCTURED GRIDS

در اين نوشتار جزييات الگوريتم روش خط جريان براي حل صريح معادله‌ي انتقال حاكم بر جريان سيال در محيط‌هاي متخلخل در شبكه‌بندي دو‌بعدي باسازمان بيان مي‌شود. با توجه به تبديل معادله‌ي انتقال از حالت دو‌بعدي به چند معادله‌ي يك‌بعدي و غيروابسته به گسسته‌سازي مكاني در روش خط جريان، هزينه‌ي محاسباتي آن به‌مراتب كم‌تر از روش‌هاي استاندارد ــ نظير اختلاف محدود و حجم محدود ــ است. با توجه به اين كه هر بار يك مسيله‌ي يك‌بعدي ذخيره و حل مي‌‌شود، اين روش در مقايسه با روش‌هاي استاندارد به حافظه‌ي كم‌تري نياز دارد. بر‌اساس نتايج شبيه سازي نشان داده مي‌شود كه اين روش براي شبيه‌سازي يك مدل با 90000 سلول محاسباتي، تقريباً 26 برابر سريع‌تر از روش‌هاي استاندارد خواهد بود. همچنين نشان داده مي‌شود كه ضريب افزايش سرعت شبيه‌سازي با افزايش تعداد سلول‌هاي فعال و ناهمگوني در مخازن افزايش خواهد يافت.

In this work, a computational algorithm is given for solving the transport equation arising from flow in two-dimensional porous media, using the streamline method. The streamline method has four vital steps. First, the two-dimensional transport equation is decoupled into multiple 1-D equations. By using a new local variable, called the time-of-flight, these equations are solved in a way that is independent from the spatial discretization. In the second step, the streamlines are traced, based on the velocity field in the computational domain, to obtain the time-of-flight grid (TOF gird) along each streamline. Cell-by-cell streamline tracing is performed by utilizing the semi-analytical Pollock algorithm from the injector (sink) cell faces to the producer (source) cell faces or vice versa. In the third step, once all streamlines are traced, and their 1-D TOF grids are constructed, the multiple 1-D equations are solved, by either analytical or numerical methods. In the fourth step, the solutions along the TOF grid are mapped into the pressure grid (primary structured mesh) in order to construct the solution of the 2-D transport equation. This step is conducted using a volumetric averaging of the streamlines passing each cell. The streamline method requires relatively lower computational time than other conventional methods, such as finite difference and finite volume methods, as it solves only several 1-D equations, and is without restriction on the time step size. In addition, this method requires less memory than other standard methods, as it needs to save and solve one of the multiple 1-D equations each time. Here, this method is employed for solving a number of 2-D test cases, including both single-phase and two-phase flow. The test cases demonstrates that the streamline method has good accuracy with a high speed-up factor. Numerical results show that using this method to simulate flow within a 2-D problem with 90000 grid cells leads to a speed-up factor of about 26. It is also shown that the speed-up factor of the streamline method will be enhanced by increasing the heterogeneity and number of grid cells.